Ammar Askar <ammar@ammaraskar.com> added the comment: Opened https://github.com/python/cpython/pull/17064 to fix this. Essentially it's a tiny little oversight in the back-porting. In the 3.7 branch, we perform an attribute lookup for `from_param` before the union checking code, so we must remember to DECREF it. This is unlike master where the attribute lookup happens after the union checking code. λ win32\python_d.exe -m test -R 3:3 test_ctypes 0:00:00 Run tests sequentially 0:00:00 [1/1] test_ctypes beginning 6 repetitions 123456 ...... == Tests result: SUCCESS == 1 test OK. Total duration: 14.8 sec Tests result: SUCCESS ---------- nosy: +ammar2 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue16575> _______________________________________