Sheet names not being recognised using "INDIRECT"

Hi all,

Just going crazy trying to sort out a problem. I am using the "INDIRECT" function (example =INDIRECT($B132&"!$ap$4") ) to pick up information from a range of 160 sheets in an excel file. All the sheets are named with a four digit alpha numeric code. For some reason sheets names R1R2 R1R4 R2R1,etc are not recognised and I get an invalid cell reference error returned. If I change the sheet names and the INDIRECT function details to match then all works ok but not with any of the above type names.

Any ideas?