[rbmj]

/**
 * Set the PWM value based on a speed.
 * 
 * This is intended to be used by speed controllers.
 * 
 * @pre SetMaxPositivePwm() called.
 * @pre SetMinPositivePwm() called.
 * @pre SetCenterPwm() called.
 * @pre SetMaxNegativePwm() called.
 * @pre SetMinNegativePwm() called.
 * 
 * @param speed The speed to set the speed controller between -1.0 and 1.0.
 */
void PWM::SetSpeed(float speed)
{
	if (StatusIsFatal()) return;
	// clamp speed to be in the range 1.0 >= speed >= -1.0
	if (speed < -1.0)
	{
		speed = -1.0;
	}
	else if (speed > 1.0)
	{
		speed = 1.0;
	}

	// calculate the desired output pwm value by scaling the speed appropriately
	INT32 rawValue;
	if (speed == 0.0)
	{
		rawValue = GetCenterPwm();
	}
	else if (speed > 0.0)
	{
		rawValue = (INT32)(speed * ((float)GetPositiveScaleFactor()) +
									((float) GetMinPositivePwm()) + 0.5);
	}
	else
	{
		rawValue = (INT32)(speed * ((float)GetNegativeScaleFactor()) +
									((float) GetMaxNegativePwm()) + 0.5);
	}

	// the above should result in a pwm_value in the valid range
	wpi_assert((rawValue >= GetMinNegativePwm()) && (rawValue <= GetMaxPositivePwm()));
	wpi_assert(rawValue != kPwmDisabled);

	// send the computed pwm value to the FPGA
	SetRaw((UINT8)rawValue);
}

What speed are you calling the function with (or some object that calls the function). If it's not x|-1.0<=x<=1.0 then that might be the cause of your issues.

Edit: The assert that's failing is the second-to-last assert