run-time error message

[Paul Ritenour]

A form that has been used for years a suddenly started
failing when new data is added. The error message
is : run-time error '-2147352567 (800200009)':
You can't assign a value to this object. When I go to
debug the code highlighted is:
studentname = Lastname + ", " + firstname + " " + middle
name. The fields lastname, firstname and middlename all
contain data and if you place the cursor over the field
the data is displayed but for some reason the studentname
field is not updated. Any help would be appreciated

Thanks

Paul Ritenour