VB Error: Can't find project or library

Hi, All,

I made a automation add-in for MS Excel.

If I calculate a cell calling a function from the COM by return, it generates a VB error:

Can't find project or library.

However, it works fine if I calculate the spreadsheet containing functions from the COM dll by F9.

Furthermore, not all the functions in the dll have this problem. Not all the computers have this problem, either.

I checked the VBA editor, there is no missing reference.

Does any one have any idea what is going on?

Thanks a lot!