The method mentioned in the title uses the "assert" statement to check the headers of a token. Unfortunately, assert statements are completely removed when starting Python with "-O" (or "-OO"). See here https://docs.python.org/3/using/cmdline.html#miscellaneous-options As a result, the OIDC connection cannot be established. Recommended fix: Use "if" statements in such cases. I found two more issues like this relevant to my usecase: * method JWS._is_compact_jws * Operator \_\_eq\_\_ in class jwk.Key Note: https://github.com/IdentityPython/pyjwkest/pull/99 fixes the first of these issues, but not the other ones.