[AccessD] Error 2448 "You can not assign a value to this object."

Folks,

I am frustrated because a form has started throwing an error and I can not
seem to isolate it. This was working last week, I think, and now I get the
error.  It is probably something very simple I am not seeing.  IN the onLoad
event of the form I have a statement 

Me.txtFee.Value = Nz(DSum("ShowAmount", "qryVendorWeddingFee"), 0) +
Nz(DSum("TotalTax", "qryVendorWeddingFee"), 0)

When the form opens I get the subject error message. For debugging purposes
I commmented the line out and put a button on the form that does the same
thing.  This works fine setting me.txtFee to the proper value.  At first I
was using me.txtFee =, and got the same error.

Do any of you see what is going on here?

Thanks in advance.

Doug