If you are starting with the idfrd function, note that the documentation states:
The 'ResponseData' property stores the frequency response data as a 3-D array of complex numbers.
Amplitudes in decibels are not complex numbers.
If you are using the iddata function, see the documentation section on Frequency-Domain Data . Note that for it ‘...the data, which consists of the complex-valued input-output frequency-domain data U and Y, frequency vector W, and sample time Ts.’
Again, amplitudes in decibels are not complex numbers.