diff --git a/moto/dynamodb2/responses.py b/moto/dynamodb2/responses.py
index 0f60285d..d34b176a 100644
--- a/moto/dynamodb2/responses.py
+++ b/moto/dynamodb2/responses.py
@@ -565,7 +565,7 @@ class DynamoHandler(BaseResponse):
         name = self.body['TableName']
         key = self.body['Key']
         return_values = self.body.get('ReturnValues', 'NONE')
-        update_expression = self.body.get('UpdateExpression')
+        update_expression = self.body.get('UpdateExpression', '').strip()
         attribute_updates = self.body.get('AttributeUpdates')
         expression_attribute_names = self.body.get(
             'ExpressionAttributeNames', {})