Unmerge All Cells

Why does this not work? Is it because the first sheet does not have merged cells? If so, how do I check to see that the sheet does not have any merged cells?

Function CleanBook()
Dim sh As Worksheet

For Each sh In ActiveWorkbook.Sheets
    sh.Cells.UnMerge
    sh.Name = Trim(sh.Name)
Next sh
End Function

I get "application-defined or object-defined error"