diff --git a/tests/test_vyper.py b/tests/test_vyper.py
index 84392e6..ea1c018 100644
--- a/tests/test_vyper.py
+++ b/tests/test_vyper.py
@@ -294,6 +294,19 @@ def test_toml(self):
         self._init_toml()
         self.assertEqual("TOML Example", self.v.get("title"))
 
+    def test_env_override_default_subclass(self):
+        os.environ['CLASS.KEY2'] = 'newval2'
+
+        self.v.set_default('class.key1', 'val1')
+        self.v.set_default('class.key2', 'val2')
+        self.v.set_default('class.key3', 'val3')
+
+        self.v.bind_env('class.key1')
+        self.v.bind_env('class.key2')
+        self.v.bind_env('class.key3')
+
+        self.assertEqual('val1', self.v.get('class.key1'))
+
     def test_env(self):
         self._init_json()
 
diff --git a/vyper/vyper.py b/vyper/vyper.py
index 985598d..3059004 100644
--- a/vyper/vyper.py
+++ b/vyper/vyper.py
@@ -321,12 +321,14 @@ def _find_insensitive(self, key, source):
         return source.get(real_key)
 
     def _set_insensitive(self, key, val, source):
-        real_key = self._find_real_key(key, source)
-        if real_key is None:
-            msg = "No case insensitive variant of {0} found.".format(key)
-            raise KeyError(msg)
+        if source:
+            real_key = self._find_real_key(key, source)
+            if real_key is None:
+                msg = "No case insensitive variant of {0} found.".format(key)
+                raise KeyError(msg)
 
-        source[real_key] = val
+            source[real_key] = val
+            return True
 
     def _find(self, key):
         """Given a key, find the value
@@ -385,8 +387,8 @@ def _find(self, key):
                         real_key = self._find_real_key(path, temp)
                         temp = temp[real_key]
 
-                    self._set_insensitive(item["final_key"], val, temp)
-                    found_in_env = True
+                    if self._set_insensitive(item["final_key"], val, temp):
+                        found_in_env = True
                 else:
                     log.debug("{0} env value unset".format(item["env_key"]))