Yes exactly, but for some reason the background checks are still happening and a value is still expected.
If you supply any value in your incoming idoc it should get past this error and then simply not load which is fine?
Have a look at the value table for this field and pick any valid value. Put this value in your inbound idoc to test it. Run it and see if it processes successfully. Then check to make sure that it didn't actually load into the table on the target system. If it has loaded and actually you don't want a value then have a look at your value table to see if a blank is acceptable in which case then try with a blank.