diff --git a/vyper/utils.py b/vyper/utils.py
index f4f14a346e..3f19a9d15c 100644
--- a/vyper/utils.py
+++ b/vyper/utils.py
@@ -25,9 +25,10 @@ class OrderedSet(Generic[_T]):
     """
 
     def __init__(self, iterable=None):
-        self._data = dict()
-        if iterable is not None:
-            self.update(iterable)
+        if iterable is None:
+            self._data = dict()
+        else:
+            self._data = dict.fromkeys(iterable)
 
     def __repr__(self):
         keys = ", ".join(repr(k) for k in self)
@@ -57,6 +58,7 @@ def pop(self):
     def add(self, item: _T) -> None:
         self._data[item] = None
 
+    # NOTE to refactor: duplicate of self.update()
     def addmany(self, iterable):
         for item in iterable:
             self._data[item] = None
@@ -109,11 +111,11 @@ def intersection(cls, *sets):
         if len(sets) == 0:
             raise ValueError("undefined: intersection of no sets")
 
-        ret = sets[0].copy()
-        for e in sets[0]:
-            if any(e not in s for s in sets[1:]):
-                ret.remove(e)
-        return ret
+        tmp = sets[0]._data.keys()
+        for s in sets[1:]:
+            tmp &= s._data.keys()
+
+        return cls(tmp)
 
 
 class StringEnum(enum.Enum):