Value $Sys_Def$ for the flexfield segment does not exist in the value set

We are using an SIT to store our employees education information. I have added a new segment, Degree Policy, to the flexfield. The valid values for this segment are housed in a value set titled HR_DEGREE_POLICY. This segment is not a required field. However, when I go to some, not all, records I am presented with the error below:

Value $Sys_Def$ for the flexfield segment Degree Policy does not exist in the value set HR_DEGREE_POLICY.

The error is present on records where the Degree Policy value is null and on records where the value contains data from the value set. The error does not appear on all records.