AM335x SPI slave mode TX problem

I'm having an intermittent problem running the BeagleBone Black's SPI1 in slave mode.  I have designed all my TX messages to end in 0, and have verified every way I know how that the last byte written to MCSPI_TX0 is 0.  But after a few minutes of running, there is a non-zero value stuck on the output.  It is usually 0x80, (I am in 8-bit mode), and I can clearly see the 0x80 coming out of the D1 line on a scope.  The communication is very reliable initially, several messages get through OK before the error.  I have to soft reset the AM335x SPI to clear the error condition.