A question about USRP

Hey guys,
I’m using USRPN210 with Daughterboard XCVR2450 , when i use
uhd_usrp_sink at center frequency 4.5 GHz , i get the following warning
UHD Warning:
The hardware does not support the requested TX frequency:
Target frequency: 4500.000000 MHz
Actual frequency: 5200.000000 MHz
What does that warning mean ?
please, help
Thanks in advance