HOW TO GET FFT

I need to get hte fft of a voltage, measured with voltage sensor(simscape) and then ps-to-simulink converter. The command window in matlab shows: "Warning: Reported by S-Function 'sdspfft2' in 'pwm_control_2/FFT': Computing the FFT of a scalar input."

Do I need to set some settings insode the fft block, or better use another FFT block (e.g. from simpower). For the data types do I need to specify something or leave it as is (inherit..)?

Thanks for helping