Hiya,
that is an error you ideally as a user shouldn’t see, as will be from the pact core that your language implementation uses.
the best advise would be to raise your question in the pact-xyz where channel for your specific language implementation. i imagine yours is #C9VBGLUM9
also if you let people know where version of the libraries you are, what system and what your test look likes it will help.
there could be a number of reasons, and without additional detail it’s hard to advise :)