GMSK Modulation. Errors in MATLAB

Hi!

I'm making several simulations about communication signals in CubeSATs. The objective is to use a frame (.wav) that is modulated in 2-FSK. Now, I want to use GMSK modulation to check the behavior of the signal in a AWGN channel. Moreover, I want that MATLAB can play the frame demodulated (after the channel) because I use a SDR. For this, I've written the next code:

[x, Fs] = wavread('tramaPolitech.wav');
Hmod = comm.GMSKModulator('BitInput', false, 'SamplesPerSymbol', 16);
Hdem = comm.GMSKDemodulator('BitOutput', false, 'SamplesPerSymbol', 16);

x_GSMK=step(Hmod,x(:,1));

y=awgn(x_GSMK,4.125,'measured');
u=step(Hdem,y);
sound(u,Fs);

The problem is that MATLAB give me the error:

Error using GMSKModulator/step
When the BitInput property is set to false, inputs must be in the range +/- (ModulationOrder-2p-1), p=0,1, ...,
(ModulationOrder/2)-1.

x is a vector of 374850x2 (double) and its valours are between -1 and +1.

What can I do?

Thanks!