diff --git a/matlab/k_order_pert.m b/matlab/k_order_pert.m
index ee22c9964..777e5d253 100644
--- a/matlab/k_order_pert.m
+++ b/matlab/k_order_pert.m
@@ -26,19 +26,19 @@ order = options.order;
 
 switch(order)
   case 1
-    [err, g_1] = k_order_perturbation(dr,M,options, oo , ['.' ...
+    [err, g_1] = k_order_perturbation(dr,M,options, ['.' ...
                         mexext]);
     mexErrCheck('k_order_perturbation', err);
     dr.g_1 = g_1;
   case 2
-    [err, g_0, g_1, g_2] = k_order_perturbation(dr,M,options, oo , ['.' ...
+    [err, g_0, g_1, g_2] = k_order_perturbation(dr,M,options, ['.' ...
                         mexext]);
     mexErrCheck('k_order_perturbation', err);
     dr.g_0 = g_0;
     dr.g_1 = g_1;
     dr.g_2 = g_2;
   case 3
-    [err, g_0, g_1, g_2, g_3] = k_order_perturbation(dr,M,options, oo , ['.' ...
+    [err, g_0, g_1, g_2, g_3] = k_order_perturbation(dr,M,options, ['.' ...
                         mexext]);
     mexErrCheck('k_order_perturbation', err);
     dr.g_0 = g_0;
diff --git a/mex/sources/k_order_perturbation/k_order_perturbation.cc b/mex/sources/k_order_perturbation/k_order_perturbation.cc
index bb437990e..666a6549c 100644
--- a/mex/sources/k_order_perturbation/k_order_perturbation.cc
+++ b/mex/sources/k_order_perturbation/k_order_perturbation.cc
@@ -70,20 +70,19 @@ extern "C" {
   mexFunction(int nlhs, mxArray *plhs[],
               int nrhs, const mxArray *prhs[])
   {
-    if (nrhs < 5 || nlhs < 2)
-      DYN_MEX_FUNC_ERR_MSG_TXT("Must have exactly 5 input parameters and take at least 2 output parameters.");
+    if (nrhs != 4 || nlhs < 2)
+      DYN_MEX_FUNC_ERR_MSG_TXT("Must have exactly 4 input parameters and take at least 2 output parameters.");
 
     const mxArray *dr = prhs[0];
     const mxArray *M_ = prhs[1];
     const mxArray *options_ = prhs[2];
-    const mxArray *oo_ = prhs[3];
 
     mxArray *mFname = mxGetField(M_, 0, "fname");
     if (!mxIsChar(mFname))
       DYN_MEX_FUNC_ERR_MSG_TXT("Input must be of type char.");
 
     string fName = mxArrayToString(mFname);
-    const mxArray *mexExt = prhs[4];
+    const mxArray *mexExt = prhs[3];
     string dfExt = mxArrayToString(mexExt); // Dynamic file extension, e.g. ".dll" or ".mexw32"
 
     int kOrder;