Mechjeb won't throttle after ascent autopilot disengages

I'm trying to get MechJeb to manage a rendesvous with a space station. After the Ascent Autopilot disengages, I set my target to the station and activate the rendesvous module, but when it reaches the next node to start the plane correction burn, it says that it's starting the burn but the throttle stays at 0. Any other command that I put in causes it to behave normally until it reaches the point where it needs to change the throttle, but the throttle is at 0 and won't budge (and I can't adjust it manually, either). I have no joystick or other control devices plugged in.