This API call cannot be made when in I2C slave mode

I am using to Ni-8452 devices to test an interface before hardware is available,  I am using the i2C interface with one in master and one in slave mode.  But whenever I try to read or write to the digital IO lines on the slave device I get the errror "The device is in I2C slave mode. This API call cannot be made when in I2C slave mode. Refer to your documentation to determine how to stop I2C slave mode"

So does that mean I can only get I2C or digital IO?  Or is this a configuration issue.

Thanks