Modulation QPSK matlab application

i want to do the QPSK modulation,and it does not work.here's the code:

 qpskMod = comm.QPSKModulator('BitInput',true);
 U = 2*round(rand(1,R*Nc))-1;
 a=qpskMod(U)

thanks for advance

[Geoff Hayes]

Please clarify what you mean by it does not work. If you are observing an error message, then copy and paste the full error message to this question.