Fix #1318

moto/s3/responses.py

@@ -764,7 +764,7 @@ class ResponseObject(_TemplateEnvironmentMixin):
             return FakeTagging()
 
     def _tagging_from_xml(self, xml):
-        parsed_xml = xmltodict.parse(xml)
+        parsed_xml = xmltodict.parse(xml, force_list={'Tag': True})
 
         tags = []
         for tag in parsed_xml['Tagging']['TagSet']['Tag']:

tests/test_s3/test_s3.py

@@ -1775,6 +1775,30 @@ def test_boto3_put_object_tagging():
     resp['ResponseMetadata']['HTTPStatusCode'].should.equal(200)
 
 
+@mock_s3
+def test_boto3_put_object_tagging_with_single_tag():
+    s3 = boto3.client('s3', region_name='us-east-1')
+    bucket_name = 'mybucket'
+    key = 'key-with-tags'
+    s3.create_bucket(Bucket=bucket_name)
+
+    s3.put_object(
+        Bucket=bucket_name,
+        Key=key,
+        Body='test'
+    )
+
+    resp = s3.put_object_tagging(
+        Bucket=bucket_name,
+        Key=key,
+        Tagging={'TagSet': [
+            {'Key': 'item1', 'Value': 'foo'}
+        ]}
+    )
+
+    resp['ResponseMetadata']['HTTPStatusCode'].should.equal(200)
+
+
 @mock_s3
 def test_boto3_get_object_tagging():
     s3 = boto3.client('s3', region_name='us-east-1')