Build Definition - Getting " The revision number $(Rev:.rr) is allowed to occur only at the end of the format string." Error

I have configured the format as below:

"$(BuildDefinitionName)_$(Date:yyyyMMdd)_$(Rev:.rr)"

Even then after running the build, i am getting the below error:

The revision number $(Rev:.rr) is allowed to occur only at the end of the format string.

I am clueless of what can be the reason for this error.

TIA.