I have never seen this befo

In my workbook wbook.xls (Excel XP) I have several macros. On Tools | Macro
| Macros, a dialogue comes up showing the available macros. One of the macros
MyMacro is listed as 'wbook.xls'!MyMacro and selecting it does NOT enable the
Run button. The Run button does get enabled with the other macros - all of
them are in the Worksheet and are listed as ThisWorkbook.MyMacro(n).

However, after Alt + F11, I can step through MyMacro.

There is no open dialogue or error message, no formulaes in the workbook,
and there is no cell editing in progress.

I am completely baffled by this: can someone explain what is happening?

Thanks.