diff --git a/graphene/types/definitions.py b/graphene/types/definitions.py
index 4dddaa90..37fa78f5 100644
--- a/graphene/types/definitions.py
+++ b/graphene/types/definitions.py
@@ -45,8 +45,11 @@ class GrapheneEnumType(GrapheneGraphQLType, GraphQLEnumType):
                 # Try and get enum by value
                 value = enum(value)
             except ValueError:
-                # Try ang get enum by name
-                value = enum[value]
+                # Try and get enum by name
+                try:
+                    value = enum[value]
+                except KeyError:
+                    value = None
         return super(GrapheneEnumType, self).serialize(value)
 
 
diff --git a/graphene/types/tests/test_enum.py b/graphene/types/tests/test_enum.py
index 7f9fcc4a..a4994246 100644
--- a/graphene/types/tests/test_enum.py
+++ b/graphene/types/tests/test_enum.py
@@ -321,6 +321,32 @@ def test_enum_resolver_compat():
     assert results.data["colorByName"] == Color.RED.name
 
 
+def test_enum_resolver_invalid():
+    from enum import Enum as PyEnum
+
+    class Color(PyEnum):
+        RED = 1
+        GREEN = 2
+        BLUE = 3
+
+    GColor = Enum.from_enum(Color)
+
+    class Query(ObjectType):
+        color = GColor(required=True)
+
+        def resolve_color(_, info):
+            return "BLACK"
+
+    schema = Schema(query=Query)
+
+    results = schema.execute("query { color }")
+    assert results.errors
+    assert (
+        results.errors[0].message
+        == "Expected a value of type 'Color' but received: 'BLACK'"
+    )
+
+
 def test_enum_mutation():
     from enum import Enum as PyEnum