Rails null value in column error (only in test)

I created a new model and associated migration but I haven't created any instances of this model. My development server runs fine but when I run my tests every test fails with the following error:

DRb::DRbRemoteError: PG::NotNullViolation: ERROR: null value in column "some_value" of relation "table_name" violates not-null constraint

some_value is a non-null column but I haven't set a default because there isn't a sensible default and I want the field to be properly provided any time a record is created. Given that I haven't created any records and I am not running any test code using this model yet, what is causing the issue?