diff --git a/include/eigenpy/user-type.hpp b/include/eigenpy/user-type.hpp
index 0e75214f1b4075dafa23ada1c839c65c44e3c338..50fb676f038f0f1866cf87caa68b7f360e4f17be 100644
--- a/include/eigenpy/user-type.hpp
+++ b/include/eigenpy/user-type.hpp
@@ -256,6 +256,8 @@ namespace eigenpy
     
 //    PyTypeObject * to_py_type = Register::getPyType<To>();
     int to_typenum = Register::getTypeCode<To>();
+    assert(to_typenum >= 0 && "to_typenum is not valid");
+    assert(from_array_descr != NULL && "to_typenum is not valid");
     
     if(PyArray_RegisterCastFunc(from_array_descr,
                                 to_typenum,