mixer in simulink

hi all

i am trying to run the mixer in simulink
it is a downconverting mixer
the input frequency is 2GHz and LO frequency is 125 MHz
i get the error as

Error using ==> rfckt.mixer.convertfreq
Mixer output frequency is negative. Check FLO of MIXER and the simulation frequency.......

need help

thnx

-Typo's somewhere?
-are you redefining constants?
-signals input to the correct ports?
-Units correct? Make everything in Hz
-Mixer configured correctly?

Dave