Debugger error during I2C write

I'm getting the following debugger errors when trying to read an I2C register:

ARM9_0: Error: Error 0xA0002020/-140 Error during: Execution, Control,  Error generated by SD scan controller module
ICEPICK_C: Error: Error 0xE0002024/-173 Error during: Register, Execution, Control,  Error generated by SD scan controller module

From I2C_write in the CSL - I get the errors when I try to step this line:

        // Check for NACK Signal
        if(CSL_FEXT(i2c0Regs->ICSTR, I2C_ICSTR_NACK) == 1)

This is after many earlier successful reads.

Any clue? This is when trying to talk to an LTC2991.