Serial Port Overflow

I am using serial port D on an RCM3700 configured at 9600 BAUD to read serial data from a PC. The serial port cable from the PC is not always connected, and the problem occurs when the cable is disconnected. The driver chip I am using drives the RX line(PC1) to a constant low when the cable is disconnected. The problem is that the RCM3700 serial port sees this as a constant data stream of 0x00 and continuously overflows the buffer. The serial line is held low at this point – there are no transitions on it at all.
From what I gather from the documentation, the serial port will not attempt to read data until it detects the start bit transition. It does this by seeing two different values in two consecutive clocks. Immediately after that it will begin to sample the 7 or 8 data bits followed by the stop bit. So if the input serial line is held at a constant low, then how is it detecting two different values in two clocks and then reading in data?
There is not a lot of configuration needed for serial port D, but it has been verified several times. Basically the RCM3700 is detecting data when there is none. I have several boards like this, so it is not one bad RCM3700 or driver chip.

Has anyone seen this problem and found a workaround?