i am having trouble using the write to workspace block to save variables giving me an error

ok guys so i used a find peak function to find the values from my FFT waveform and the associated frequencies,the functions works as i saw the variables using disp(pks). so im currently trying to save these variables by using the to workspace function block in simulink and i got this error.( Data 'pks' is inferred as a variable size matrix, while its specified type is something else). can anyone tell me how to fix this