source evaluates to an error

[jongooligan]

Trying to use a dynamic range to validate cells on an input sheet using list = SomeName where SomeName is derived from this formula
=OFFSET(BB!$G$1,0,0,COUNTA(BB!P:P),1)
BB is a sheet that stores various named lists

When I try to validate the cells Excel returns "the source currently evaluates to an error"

There's no help available from the msgbox so can someone here help please?

BTW - if I try to validate any cells on the input sheet Excel will not allow me to select any other sheet tab.

 

[jongooligan]

OK - nothing wrong with formula, nothing wrong with the source list.
Input sheet contained column headers that had been copied and pasted from a protected sheet which had some hidden columns and this seemed to have caused the problem.
Deleted sheet and started again - it's working fine now but I'd like to know why Excel throws out that very unhelpful error message.