Invalid Use Of Property When Calling Subroutine

I've ran into a problem with one of my workbooks which appears to only affect Excel 2007. It works fine in Excel 2003.

Basically I am calling a sub routine contained in a module from worksheet code. The sub is Public in the module and I use the following code to call the sub.. The code Errors immediately here -

Private Sub mybutton_Click()
Worksheet Code -


Private Sub mybutton_Click()
mysub
End Sub
Module Code -

Public Sub mysub()
...my code
End Sub

This works fine in Excel 2003?! Is there a different way to call the sub with Excel 2007