I've had this issue too, with Australian zipcodes; I had left the start and end fields empty; I have now entered '00000' and 99999 (ie from string to number) this has solved the immediate error, but is there anywhere a guide to the correct way to configure these fields?
As far as I can tell, this error check in the code applies to USA codes only, where shipping rates might be based on the zips; could the check just be limited to 'country = USA', or are there other countries that need the same zip-based shipping rate systems?
The same error message, by the way, can be generated by wrongly configuring the weight ranges; overlapping entries, for instance, will cause it