diff --git a/graphene/relay/tests/test_mutations.py b/graphene/relay/tests/test_mutations.py
index 0874be65..b9b5dc51 100644
--- a/graphene/relay/tests/test_mutations.py
+++ b/graphene/relay/tests/test_mutations.py
@@ -56,8 +56,7 @@ def test_mutation_arguments():
     client_mutation_id_field = inner_type._meta.fields_map[
         'clientMutationId']
     assert issubclass(inner_type, graphene.InputObjectType)
-    assert isinstance(client_mutation_id_field.type, graphene.NonNull)
-    assert isinstance(client_mutation_id_field.type.of_type, graphene.String)
+    assert isinstance(client_mutation_id_field.type, graphene.String)
     assert client_mutation_id_field.object_type == inner_type
     assert isinstance(schema.T(client_mutation_id_field), GraphQLInputObjectField)
 
@@ -65,7 +64,7 @@ def test_mutation_arguments():
 def test_execute_mutations():
     query = '''
     mutation M{
-      first: changeNumber(input: {clientMutationId: "mutation1"}) {
+      first: changeNumber(input: {}) {
         clientMutationId
         result
       },
@@ -81,7 +80,7 @@ def test_execute_mutations():
     '''
     expected = {
         'first': {
-            'clientMutationId': 'mutation1',
+            'clientMutationId': None,
             'result': '1',
         },
         'second': {
diff --git a/graphene/relay/types.py b/graphene/relay/types.py
index e1a1ffc5..44837e55 100644
--- a/graphene/relay/types.py
+++ b/graphene/relay/types.py
@@ -68,7 +68,7 @@ class Node(six.with_metaclass(NodeMeta, Interface)):
 
 
 class MutationInputType(InputObjectType):
-    clientMutationId = String(required=True)
+    clientMutationId = String()
 
 
 class RelayMutationMeta(MutationMeta):
@@ -90,7 +90,7 @@ class RelayMutationMeta(MutationMeta):
 
 
 class ClientIDMutation(six.with_metaclass(RelayMutationMeta, Mutation)):
-    clientMutationId = String(required=True)
+    clientMutationId = String()
 
     class Meta:
         abstract = True