UDF works in 2003, but fails in 2007

I have a user defined function which works in Excell 2003, but fails in Excel 2007.

Function IsFormula(Check_Cell As Range)
IsFormula = Check_Cell.HasFormula
End Function

I have some code which copies a range from another workbook and then pastes this into the open workbook.
The user can choose whether to paste the data as "Values" or as "Link".
There is a cell in the worksheet which uses the "IsFormula" function to check the first cell of the pasted range to see if it contains Data (i.e. it is pasted values) or a Formula (i.e. it is linked) and then display a message to let the user know that they are currently using linked data or pasted values.
The whole thing worked perfectly in Excel 2003, but in Excel 2007 when the range is pasted, the "IsFormula" function gives a "#VALUE!" error. At this point F9 (recalc) sometimes cures the problem, but not always.
Copying the data in the referenced cell to another cell, then copying back again also cures the problem.

Any explanation of what is going on and how to solve it would be much appreciated.
TIA
Peter Ross