Suspected integer underflow when switch returns a negative number

I am creating a house set which uses the construction_check callback to allow or deny construction based on a land value I calculate from various tile attributes. If I use a negative modifier, say a switch which checks if the tile is desert and returns return: -2;, the house is allowed to build even if I require a value of >=10000. As -2 is obviously not >= 10000, I believe this is an integer underflow caused by returning a negative value.

I would be happy to build a simple GRF to demonstrate this, if requested.

The expected behavior, assuming a switch to a signed integer isn't feasible, would be for NMLC to return an error if the user attempts to return a negative number.

Start by reproducing the construction_check callback with a negative return value, such as -2, and a required land value of >=10000. Trace how NMLC handles the callback's return value and verify that a negative result is rejected with an error, or that the comparison behaves correctly for signed values.