Crashes when calling subroutine

Hi,
I have three different modules, A,B,C. In A I have defined some global variables. In module B I have a subroutine M.

Sub M
Call C.test
End sub

In module C I have a subroutine test

Sub test
a=1
End sub

When I try to run subroutine M to call test it crashes. What is wrong?

Stefan