--- a/taskcluster/taskgraph/loader/test.py
+++ b/taskcluster/taskgraph/loader/test.py
@@ -117,17 +117,17 @@ def get_test_platforms(test_platforms_cf
 def expand_tests(test_sets_cfg, test_platforms):
     """Expand the test sets in `test_platforms` out to sets of test names.
     Returns a dictionary like `get_test_platforms`, with an additional
     `test-names` key for each test platform, containing a set of test
     names."""
     rv = {}
     for test_platform, cfg in test_platforms.iteritems():
         test_sets = cfg['test-sets']
-        if not set(test_sets) < set(test_sets_cfg):
+        if not set(test_sets) <= set(test_sets_cfg):
             raise Exception(
                 "Test sets {} for test platform {} are not defined".format(
                     ', '.join(test_sets), test_platform))
         test_names = set()
         for test_set in test_sets:
             test_names.update(test_sets_cfg[test_set])
         rv[test_platform] = cfg.copy()
         rv[test_platform]['test-names'] = test_names