--- a/testing/tools/autotry/autotry.py
+++ b/testing/tools/autotry/autotry.py
@@ -33,17 +33,19 @@ class AutoTry(object):
         self.mach_context = mach_context
 
         if os.path.exists(os.path.join(self.topsrcdir, '.hg')):
             self._use_git = False
         else:
             self._use_git = True
 
     def resolve_manifests(self, paths=None, tags=None):
-        assert tags or paths
+        if not paths or tags:
+            return {}
+
         tests = list(self.resolver.resolve_tests(tags=tags,
                                                  paths=paths,
                                                  cwd=self.mach_context.cwd))
         manifests_by_flavor = defaultdict(set)
 
         for t in tests:
             if t['flavor'] in AutoTry.test_flavors:
                 flavor = t['flavor']