diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 2b1a1b5ee..af67f5091 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -425,12 +425,17 @@ class DynamoDBBackend(BaseBackend):
     def update_item(self, table_name, key, update_expression, attribute_updates):
         table = self.get_table(table_name)
 
-        if table.hash_key_attr in key:
-            # Sometimes the key is wrapped in a dict with the key name
-            key = key[table.hash_key_attr]
+        if all([table.hash_key_attr in key, table.range_key_attr in key]):
+            hash_value = DynamoType(key[table.hash_key_attr])
+            range_value = DynamoType(key[table.range_key_attr])
+        elif table.hash_key_attr in key:
+            hash_value = DynamoType(key[table.hash_key_attr])
+            range_value = None
+        else:
+            hash_value = DynamoType(key)
+            range_value = None
 
-        hash_value = DynamoType(key)
-        item = table.get_item(hash_value)
+        item = table.get_item(hash_value, range_value)
         if update_expression:
             item.update(update_expression)
         else: