diff --git a/mitogen/master.py b/mitogen/master.py
index efadfffb..be92d280 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -661,7 +661,7 @@ class ModuleFinder(object):
         while stack:
             name = stack.pop(0)
             names = self.find_related_imports(name)
-            stack.extend(set(names).difference(found, stack))
+            stack.extend(set(names).difference(set(found).union(stack)))
             found.update(names)
 
         found.discard(fullname)