From c308ef4364aa935afe9c50474f54fc8e59922cac Mon Sep 17 00:00:00 2001
From: Steve Pulec <spulec@gmail.com>
Date: Sun, 1 May 2016 22:34:16 -0400
Subject: [PATCH] Fix update_auto_scaling_group. Closes #596.

---
 moto/autoscaling/models.py                 |  5 +++--
 tests/test_autoscaling/test_autoscaling.py | 22 +++++++++++++++++++++-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/moto/autoscaling/models.py b/moto/autoscaling/models.py
index eef22b397..bf4c42fd7 100644
--- a/moto/autoscaling/models.py
+++ b/moto/autoscaling/models.py
@@ -221,8 +221,9 @@ class FakeAutoScalingGroup(object):
         self.max_size = max_size
         self.min_size = min_size
 
-        self.launch_config = self.autoscaling_backend.launch_configurations[launch_config_name]
-        self.launch_config_name = launch_config_name
+        if launch_config_name:
+            self.launch_config = self.autoscaling_backend.launch_configurations[launch_config_name]
+            self.launch_config_name = launch_config_name
         self.vpc_zone_identifier = vpc_zone_identifier
         self.health_check_period = health_check_period
         self.health_check_type = health_check_type
diff --git a/tests/test_autoscaling/test_autoscaling.py b/tests/test_autoscaling/test_autoscaling.py
index 6a5f768bf..7ee9b30f0 100644
--- a/tests/test_autoscaling/test_autoscaling.py
+++ b/tests/test_autoscaling/test_autoscaling.py
@@ -418,4 +418,24 @@ def test_describe_autoscaling_groups():
             AutoScalingGroupNames=["test_asg"]
         )
         response['ResponseMetadata']['HTTPStatusCode'].should.equal(200)
-        response['AutoScalingGroups'][0]['AutoScalingGroupName'].should.equal('test_asg')
\ No newline at end of file
+        response['AutoScalingGroups'][0]['AutoScalingGroupName'].should.equal('test_asg')
+
+@mock_autoscaling
+def test_update_autoscaling_group_boto3():
+    client = boto3.client('autoscaling', region_name='us-east-1')
+    _ = client.create_launch_configuration(
+        LaunchConfigurationName='test_launch_configuration'
+    )
+    _ = client.create_auto_scaling_group(
+        AutoScalingGroupName='test_asg',
+        LaunchConfigurationName='test_launch_configuration',
+        MinSize=0,
+        MaxSize=20,
+        DesiredCapacity=5
+    )
+
+    response = client.update_auto_scaling_group(
+        AutoScalingGroupName='test_asg',
+        MinSize=1,
+        DesiredCapacity=1,
+    )