Linking .Net fieldnames error

I am linking a function to .Net by creating .dll file. The code works correctly on Matlab but, when I am trying to use the function I get this error: "An unhandled exception of type 'System.Exception' occurred in MWArray.dll

Additional information:

... MWMCR::EvaluateFunction error ...

Undefined function 'fieldnames' for input arguments of type 'cell'."

The .net code is simply calling the function with no arguments

Using the debugger on matlab raw features holds 3 named structs and works well on matlab