diff --git a/src/dynamic-graph-py.cc b/src/dynamic-graph-py.cc
index 6976c16adc0aee2a4275fcdc46e635f15543c9e6..38874204ed41cddb25bf6c572261adaed1653e02 100644
--- a/src/dynamic-graph-py.cc
+++ b/src/dynamic-graph-py.cc
@@ -56,10 +56,18 @@ namespace dynamicgraph {
       if (!PyArg_ParseTuple(args,"OO", &objOut, &objIn))
 	return NULL;
 
-      if (!PyCObject_Check(objOut))
+      if (!PyCObject_Check(objOut)) {
+	PyErr_SetString(PyExc_TypeError,
+			"first argument should be a pointer to"
+			" signalBase<int>.");
 	return NULL;
-      if (!PyCObject_Check(objIn))
+      }
+      if (!PyCObject_Check(objIn)) {
+	PyErr_SetString(PyExc_TypeError,
+			"second argument should be a pointer to"
+			" signalBase<int>.");
 	return NULL;
+      }
 
       pObjIn = PyCObject_AsVoidPtr(objIn);
 	SignalBase<int>* signalIn = (SignalBase<int>*)pObjIn;