I noticed that passing keyword arguments to types.GenericAlias's __new__ causes the interpreter to hard crash and exit due to an assertion failure: import types types.GenericAlias(bad=float) Result: Assertion failed: PyTuple_CheckExact(kwnames), file ....\cpython\Python\getargs.c, line 2767 A simple fix is to just use _PyArg_NoKeywords instead of _PyArg_NoKwnames. Looking through the rest of the C code, it seems that apart from GenericAlias, only vectorcalls for various builtins use _PyArg_NoKwnames. However, they don't seem to be affected by the bug.