Help CStrings in Debug build?

Having a problem with CStrings in debug mode.

Program code works fine in release mode but when tracing a debug build CStrings seem to get garbled when using members like toupper or code like:
CString sTest;
sTest = "Testing One";
sTest = sTest + " Two";
sTest now contains "YYYYYYYYYYYYYYYYYYYY" but only in debug build!

Is it my settings ?

Thanks.

Are you passing the CString out of a function by any chance?