Prime 3.0 Custom Functions (string arguments)

I have a user function for Mathcad 15 that works great (in Mathcad 15).  I'm attempting to build that .dll for Mathcad Prime 3.0.  After compiling the code (linking the mcaduser.lib that is distributed with Mathcad Prime 3.0), all the custom functions defined in my .dll work fine in Mathcad Prime 3.0, except those which return a string type value.  Any function which returns a string will crash Mathcad Prime 3.0 when the function is called.   Even the most simple test cases, such as the following function:

LRESULT testFunction(LPMCSTRING ParamValue, LPCMCSTRING ParamName)

{

ParamValue->str = "foo";

return 0;

}

will crash Prime 3.0 when invoked in a worksheet.

Anyone else experience this issue?  Thanks for the feedback!

Scott