diff --git a/runtests.py b/runtests.py
index c340b55d8..98f34c067 100755
--- a/runtests.py
+++ b/runtests.py
@@ -45,5 +45,7 @@ if __name__ == "__main__":
             # `runtests.py TestCase [flags]`
             # `runtests.py test_function [flags]`
             pytest_args = ['tests', '-k', pytest_args[0]] + pytest_args[1:]
+    else:
+        pytest_args = []
 
     sys.exit(pytest.main(pytest_args))