Modules & VBA :: SQL To Copy Field Contents With Reserved Characters

I've got some code that loops through a record and copies the non-null fields to another record. This is the partial code:

For Each fld In tdf.Fields
If Not IsNull(Forms!contact_lookup![Contact_sub subform1].Form(fld.NAME)) Then
strSql = "UPDATE tbl1 SET tbl1.[" & fld.NAME & "] = '" & Forms!contact_lookup![Contact_sub subform1].Form(fld.NAME) & "' " & _
"WHERE tbl1.[FC_APN] = '" & Me.txtApn & "';"
DoCmd.RunSQL strSql
End If
Next fld

This works fine until it encounters a field whose contents has a reserved character, like a single quote (" ' ") in it, for example, "What's the what." At that point the SQL thinks there's a " ' " missing in the statement, and I get an error 3075 (syntax error: missing operator).