It sounds as though either frm or op1 does not exist. However, to have got
this far, frm presumably does exist, so that leaves op1 as suspect.

Do you have Option Explicit set  and have you done a compile? It will pick
up most instances of missing items.

When it fails, you will be able to look at frm and op1 in the debugger and
see that they exist.

Check that it is called op1 and that it is indeed in form  frm.