The error was coming from one of the embedded libraries.
Recent versions of wow have become stricter with what arguments can be passed to some methods.
Previously some methods getting empty or invalid arguments passed in, silently ignored them / reverted to a default behavior.
Now a lot of the interface code has moved to a more strict paradigm, where methods error if they don't get the exact arguments expected.
diff shows the breaking change.
I made a couple other small improvements to the versioning logic but addon would work without them.