How is interp & decimation in grc taking zero

Hi,

Am using grc, to create a signal source connected to USRP2 sink, and
from another USRP2 as source and to a scope sink. While setting the
parameters, i could even set to interp on USRP2 sink and decimation on
USRP2 source to zero. to see the samples or signal source correctly
recieving. But when i try using tx_samples.cc and
rx_streaming_samples.cc, it is not allowing to set to zero (only allowed
to 4 to 512).
Now my question is the usrp2 is calling the same low level C++ code and
Why is it so.

best regards,
srinivas.