fix: Add check of credentials

semantic_release/pypi.py

@@ -2,6 +2,8 @@
 """
 from invoke import run
 
+from semantic_release import ImproperConfigurationError
+
 
 def upload_to_pypi(
         dists: str = 'sdist bdist_wheel',
@@ -15,6 +17,8 @@ def upload_to_pypi(
     :param username: PyPI account username string
     :param password: PyPI account password string
     """
+    if username is None or password is None:
+        raise ImproperConfigurationError('Missing credentials for uploading')
     run('python setup.py {}'.format(dists))
     run(
         'twine upload -u {} -p {} {} {}'.format(