diff --git a/src/duneevaluate.cc b/src/duneevaluate.cc
index aa97d4e9177a1000f565042aed73df1695c2b790..ec35d9ff28233ac875bf772818e5678f9fde0ede 100644
--- a/src/duneevaluate.cc
+++ b/src/duneevaluate.cc
@@ -15,6 +15,7 @@ DEFUN_DLD(duneevaluate, args, nargout, "duneevaluate(y)\n\
 \n\
 Evaluate x -> 1/2<Ax,x> - <b,x> + H(|x|) at y using DUNE\n") {
   assert(args.length() == 1);
+  assert(nargout <= 1);
   ColumnVector point(args(0).matrix_value());
 
   int const dim = 2;
diff --git a/src/duneminimise.cc b/src/duneminimise.cc
index dc0d05a368502098799a60f25bfcc63174e7ed55..b7cc8a288da67d9cfb8e7222ae00dcc3dfd942b9 100644
--- a/src/duneminimise.cc
+++ b/src/duneminimise.cc
@@ -15,6 +15,7 @@ DEFUN_DLD(duneminimise, args, nargout, "duneminimise(y)\n\
 \n\
 Make a minimisation step of x -> 1/2<Ax,x> - <b,x> + H(|x|) using DUNE starting at x\n") {
   assert(args.length() == 1);
+  assert(nargout <= 1);
   ColumnVector current(args(0).matrix_value());
 
   int const dim = 2;