xlrd Excel script converting “#N/A” to 42

Question

I have a script that pulls data out of an excel spreadsheet using the xlrd module, specifically the row_values() method. It appears to do a great job, except for where "#N/A" has been auto-generated by previous VLookups, in which case xlrd gets "#N/A" as integer 42.

I had a look at string formatting methods but couldn't see how that was the issue.

Apart from having a script that has discovered the meaning of life (42), can anyone suggest what the issue may be?

Cheers

Note: The sheet no longer has the Vlookups in it, all values have been copied from other sheets, everything is plain values, no formulas.