Object is definitely null, and typically we don't think of objects as being operands to arithmetic operations (whereas we do when it comes to primitives). Would be nice if we could be more precise on the error message.

Two orthogonal issues:

First, we could do extra work to omit the word possibly when the type is exactly null or undefined. However, it is very rare for someone to explicitly use null or undefined as an operand, so it's not clear that it really matters all that much. This is the type of error that only surfaces in a test.

Second, we could consider using the word value instead of object when reporting for expression operands--but it's really the same error that we use for obj.foo where obj may be null or undefined. The downside would be error message searchability, but since the error is pretty obvious that may not be an issue.