--- a/run-tests
+++ b/run-tests
@@ -21,16 +21,26 @@ EXTDIR = os.path.join(HERE, 'hgext')
 
 if __name__ == '__main__':
     if 'VIRTUAL_ENV' not in os.environ:
         activate = os.path.join(HERE, 'venv', 'bin', 'activate_this.py')
         execfile(activate, dict(__file__=activate))
         sys.executable = os.path.join(HERE, 'venv', 'bin', 'python')
         os.environ['VIRTUAL_ENV'] = os.path.join(HERE, 'venv')
 
+    venv_name = os.path.relpath(os.environ['VIRTUAL_ENV'],
+                                os.path.join(HERE, 'venv'))
+
+    if venv_name == '.':
+        venv_name = 'global'
+
+    if venv_name not in ('global', 'vcssync'):
+        print('unknown virtualenv: %s (this should not happen)' % venv_name)
+        sys.exit(1)
+
     try:
         import vcttesting.docker as vctdocker
     except ImportError:
         vctdocker = None
 
     from vcttesting.testing import (
         get_docker_state,
         get_extensions,