fft function not working in .net assembly

I'm working on a signal processing function. I want to call my function from a c# application, but the matlab function fft does not work, it throws an error: An unhandled exception of type 'System.Exception' occurred in MWArray.dll Additional information:

... MWMCR::EvaluateFunction error ...

Integers can only be combined with integers of the same class, or scalar doubles.

The error is generated when the function fft is called.

Any solution for this problem?