Integration of MATLAB functions with more than one variable

Hi,
I want to realize an integration in COMSOL 4.3a using self-written MATLAB functions.

If I use a test function with one input and one output argument everything works as expected (plotting function, integration).
If I now use a function with two arguments I still can plot the function with respect to x and y, but I will get an error when trying to do an integration:
"Evalutation of result expression - Unknown function or operator."

As I said I can plot the function correctly, so it can be found. And on the other hand functions are also found in integration when using a function with one argument.

Any ideas what could be wrong and what the solution could be?

Best,
Erik