Sychronize acombo box with recordset on form

I'm using Access 2003 and Windows XP.

I have a form which I use for entering new users. On it I have a combo box which I want to use for quickly finding and editing other users records contained in the form's recordset. I have used this procedure successfully on other forms without any problem, but for some reason I keep getting the following error every time I try to use the combo box to find a user:

Runtime Error '3070':
The Microsoft Jet database engine does not recognize "royboy" as a valid field name or expression.

Following is the code I have attached to the AfterUpdate event for the combo box.

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On another forms where I have done something similar with a combo box, the field I was sychronizing with was a number field. As you can see, the strUserId field on this form is a text field. Is that the problem? If so, how do I work around it? Any help would be appreciated.

Thanks