diff --git a/src/DynamicModel.cc b/src/DynamicModel.cc
index d200e06f..15daa76c 100644
--- a/src/DynamicModel.cc
+++ b/src/DynamicModel.cc
@@ -3119,12 +3119,13 @@ DynamicModel::updateVarAndTrendModel() const
 {
   for (bool var : { true, false })
     {
-      map<string, vector<int>> trend_varr;
+      map<string, vector<optional<int>>> trend_varr;
       map<string, vector<set<pair<int, int>>>> rhsr;
       for (const auto &[model_name, eqns] : (var ? var_model_table.getEqNums()
                                              : trend_component_model_table.getEqNums()))
         {
-          vector<int> lhs, trend_var, trend_lhs;
+          vector<int> lhs, trend_lhs;
+          vector<optional<int>> trend_var;
           vector<set<pair<int, int>>> rhs;
 
           if (!var)
@@ -3160,25 +3161,25 @@ DynamicModel::updateVarAndTrendModel() const
                     catch (...)
                       {
                       }
-                  int trend_var_symb_id = equations[eqn]->arg2->findTargetVariable(lhs_symb_id);
-                  if (trend_var_symb_id >= 0)
+                  optional<int> trend_var_symb_id = equations[eqn]->arg2->findTargetVariable(lhs_symb_id);
+                  if (trend_var_symb_id)
                     {
-                      if (symbol_table.isDiffAuxiliaryVariable(trend_var_symb_id))
+                      if (symbol_table.isDiffAuxiliaryVariable(*trend_var_symb_id))
                         try
                           {
-                            trend_var_symb_id = symbol_table.getOrigSymbIdForAuxVar(trend_var_symb_id);
+                            trend_var_symb_id = symbol_table.getOrigSymbIdForAuxVar(*trend_var_symb_id);
                           }
                         catch (...)
                           {
                           }
-                      if (find(trend_lhs.begin(), trend_lhs.end(), trend_var_symb_id) == trend_lhs.end())
+                      if (find(trend_lhs.begin(), trend_lhs.end(), *trend_var_symb_id) == trend_lhs.end())
                         {
                           cerr << "ERROR: trend found in trend_component equation #" << eqn << " ("
-                               << symbol_table.getName(trend_var_symb_id) << ") does not correspond to a trend equation" << endl;
+                               << symbol_table.getName(*trend_var_symb_id) << ") does not correspond to a trend equation" << endl;
                           exit(EXIT_FAILURE);
                         }
                     }
-                  trend_var.push_back(trend_var_symb_id);
+                  trend_var.push_back(move(trend_var_symb_id));
                 }
             }
 
diff --git a/src/ExprNode.cc b/src/ExprNode.cc
index 35738af6..3107f441 100644
--- a/src/ExprNode.cc
+++ b/src/ExprNode.cc
@@ -643,10 +643,10 @@ NumConstNode::findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes)
 {
 }
 
-int
+optional<int>
 NumConstNode::findTargetVariable(int lhs_symb_id) const
 {
-  return -1;
+  return nullopt;
 }
 
 expr_t
@@ -1593,13 +1593,13 @@ VariableNode::findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes)
     datatree.getLocalVariable(symb_id)->findUnaryOpNodesForAuxVarCreation(nodes);
 }
 
-int
+optional<int>
 VariableNode::findTargetVariable(int lhs_symb_id) const
 {
   if (get_type() == SymbolType::modelLocalVariable)
     return datatree.getLocalVariable(symb_id)->findTargetVariable(lhs_symb_id);
 
-  return -1;
+  return nullopt;
 }
 
 expr_t
@@ -3393,7 +3393,7 @@ UnaryOpNode::findDiffNodes(lag_equivalence_table_t &nodes) const
   nodes[lag_equiv_repr][index] = const_cast<UnaryOpNode *>(this);
 }
 
-int
+optional<int>
 UnaryOpNode::findTargetVariable(int lhs_symb_id) const
 {
   return arg->findTargetVariable(lhs_symb_id);
@@ -5310,14 +5310,14 @@ BinaryOpNode::findTargetVariableHelper1(int lhs_symb_id, int rhs_symb_id) const
   return false;
 }
 
-int
+optional<int>
 BinaryOpNode::findTargetVariableHelper(const expr_t arg1, const expr_t arg2,
                                        int lhs_symb_id) const
 {
   set<int> params;
   arg1->collectVariables(SymbolType::parameter, params);
   if (params.size() != 1)
-    return -1;
+    return nullopt;
 
   set<pair<int, int>> endogs;
   arg2->collectDynamicVariables(SymbolType::endogenous, endogs);
@@ -5331,18 +5331,18 @@ BinaryOpNode::findTargetVariableHelper(const expr_t arg1, const expr_t arg2,
       else if (findTargetVariableHelper1(lhs_symb_id, endogs.rbegin()->first))
         return endogs.begin()->first;
     }
-  return -1;
+  return nullopt;
 }
 
-int
+optional<int>
 BinaryOpNode::findTargetVariable(int lhs_symb_id) const
 {
-  int retval = findTargetVariableHelper(arg1, arg2, lhs_symb_id);
-  if (retval < 0)
+  optional<int> retval = findTargetVariableHelper(arg1, arg2, lhs_symb_id);
+  if (!retval)
     retval = findTargetVariableHelper(arg2, arg1, lhs_symb_id);
-  if (retval < 0)
+  if (!retval)
     retval = arg1->findTargetVariable(lhs_symb_id);
-  if (retval < 0)
+  if (!retval)
     retval = arg2->findTargetVariable(lhs_symb_id);
   return retval;
 }
@@ -6447,13 +6447,13 @@ TrinaryOpNode::findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes)
   arg3->findUnaryOpNodesForAuxVarCreation(nodes);
 }
 
-int
+optional<int>
 TrinaryOpNode::findTargetVariable(int lhs_symb_id) const
 {
-  int retval = arg1->findTargetVariable(lhs_symb_id);
-  if (retval < 0)
+  optional<int> retval = arg1->findTargetVariable(lhs_symb_id);
+  if (!retval)
     retval = arg2->findTargetVariable(lhs_symb_id);
-  if (retval < 0)
+  if (!retval)
     retval = arg3->findTargetVariable(lhs_symb_id);
   return retval;
 }
@@ -6871,14 +6871,14 @@ AbstractExternalFunctionNode::findUnaryOpNodesForAuxVarCreation(lag_equivalence_
     argument->findUnaryOpNodesForAuxVarCreation(nodes);
 }
 
-int
+optional<int>
 AbstractExternalFunctionNode::findTargetVariable(int lhs_symb_id) const
 {
   for (auto argument : arguments)
-    if (int retval = argument->findTargetVariable(lhs_symb_id);
-        retval >= 0)
+    if (optional<int> retval = argument->findTargetVariable(lhs_symb_id);
+        retval)
       return retval;
-  return -1;
+  return nullopt;
 }
 
 expr_t
@@ -8391,10 +8391,10 @@ SubModelNode::findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes)
 {
 }
 
-int
+optional<int>
 SubModelNode::findTargetVariable(int lhs_symb_id) const
 {
-  return -1;
+  return nullopt;
 }
 
 expr_t
diff --git a/src/ExprNode.hh b/src/ExprNode.hh
index 9ea42414..3a77d815 100644
--- a/src/ExprNode.hh
+++ b/src/ExprNode.hh
@@ -25,6 +25,7 @@
 #include <vector>
 #include <ostream>
 #include <functional>
+#include <optional>
 
 using namespace std;
 
@@ -620,7 +621,7 @@ public:
   //! Substitute pac_target_nonstationary operator
   virtual expr_t substitutePacTargetNonstationary(const string &name, expr_t subexpr) = 0;
 
-  virtual int findTargetVariable(int lhs_symb_id) const = 0;
+  virtual optional<int> findTargetVariable(int lhs_symb_id) const = 0;
 
   //! Add ExprNodes to the provided datatree
   virtual expr_t clone(DataTree &datatree) const = 0;
@@ -811,7 +812,7 @@ public:
   expr_t substituteVarExpectation(const map<string, expr_t> &subst_table) const override;
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -884,7 +885,7 @@ public:
   expr_t substituteVarExpectation(const map<string, expr_t> &subst_table) const override;
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -989,7 +990,7 @@ public:
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   bool createAuxVarForUnaryOpNode() const;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -1096,8 +1097,8 @@ public:
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
   bool findTargetVariableHelper1(int lhs_symb_id, int rhs_symb_id) const;
-  int findTargetVariableHelper(const expr_t arg1, const expr_t arg2, int lhs_symb_id) const;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariableHelper(const expr_t arg1, const expr_t arg2, int lhs_symb_id) const;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -1232,7 +1233,7 @@ public:
   expr_t substituteVarExpectation(const map<string, expr_t> &subst_table) const override;
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -1342,7 +1343,7 @@ public:
   expr_t substituteVarExpectation(const map<string, expr_t> &subst_table) const override;
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substitutePacExpectation(const string &name, expr_t subexpr) override;
@@ -1520,7 +1521,7 @@ public:
   expr_t substituteModelLocalVariables() const override;
   void findDiffNodes(lag_equivalence_table_t &nodes) const override;
   void findUnaryOpNodesForAuxVarCreation(lag_equivalence_table_t &nodes) const override;
-  int findTargetVariable(int lhs_symb_id) const override;
+  optional<int> findTargetVariable(int lhs_symb_id) const override;
   expr_t substituteDiff(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   expr_t substituteUnaryOpNodes(const lag_equivalence_table_t &nodes, subst_table_t &subst_table, vector<BinaryOpNode *> &neweqs) const override;
   void computeSubExprContainingVariable(int symb_id, int lag, set<expr_t> &contain_var) const override;
diff --git a/src/SubModel.cc b/src/SubModel.cc
index ccc72512..f80b3b47 100644
--- a/src/SubModel.cc
+++ b/src/SubModel.cc
@@ -83,7 +83,7 @@ TrendComponentModelTable::setRhs(map<string, vector<set<pair<int, int>>>> rhs_ar
 }
 
 void
-TrendComponentModelTable::setTargetVar(map<string, vector<int>> target_vars_arg)
+TrendComponentModelTable::setTargetVar(map<string, vector<optional<int>>> target_vars_arg)
 {
   target_vars = move(target_vars_arg);
 }
@@ -319,8 +319,8 @@ TrendComponentModelTable::writeOutput(const string &basename, ostream &output) c
           i++;
         }
       output << "M_.trend_component." << name << ".target_vars = [";
-      for (auto it : target_vars.at(name))
-        output << (it >= 0 ? symbol_table.getTypeSpecificID(it) + 1 : -1) << " ";
+      for (const optional<int> &it : target_vars.at(name))
+        output << (it ? symbol_table.getTypeSpecificID(*it) + 1 : -1) << " ";
       output << "];" << endl;
 
       vector<string> target_eqtags_vec = target_eqtags.at(name);
diff --git a/src/SubModel.hh b/src/SubModel.hh
index af19c6db..6c24d2cb 100644
--- a/src/SubModel.hh
+++ b/src/SubModel.hh
@@ -24,6 +24,7 @@
 #include <map>
 #include <vector>
 #include <iostream>
+#include <optional>
 
 #include "ExprNode.hh"
 #include "SymbolTable.hh"
@@ -49,7 +50,7 @@ private:
   map<string, vector<set<pair<int, int>>>> rhs;
   map<string, vector<bool>> diff;
   map<string, vector<expr_t>> lhs_expr_t;
-  map<string, vector<int>> target_vars;
+  map<string, vector<optional<int>>> target_vars;
   map<string, map<tuple<int, int, int>, expr_t>> AR; // name -> (eqn, lag, lhs_symb_id) -> expr_t
   /* Note that A0 in the trend-component model context is not the same thing as
      in the structural VAR context. */
@@ -89,7 +90,7 @@ public:
   void setMaxLags(map<string, vector<int>> max_lags_arg);
   void setDiff(map<string, vector<bool>> diff_arg);
   void setOrigDiffVar(map<string, vector<int>> orig_diff_var_arg);
-  void setTargetVar(map<string, vector<int>> target_vars_arg);
+  void setTargetVar(map<string, vector<optional<int>>> target_vars_arg);
   void setAR(map<string, map<tuple<int, int, int>, expr_t>> AR_arg);
   void setA0(map<string, map<tuple<int, int>, expr_t>> A0_arg,
              map<string, map<tuple<int, int>, expr_t>> A0star_arg);