diff --git a/AUTHORS.md b/AUTHORS.md
index 08757d2bb..5d5b99a06 100644
--- a/AUTHORS.md
+++ b/AUTHORS.md
@@ -45,3 +45,4 @@ Moto is written by Steve Pulec with contributions from:
 * [Tom Viner](https://github.com/tomviner)
 * [Justin Wiley](https://github.com/SectorNine50)
 * [Adam Stauffer](https://github.com/adamstauffer)
+* [Guy Templeton](https://github.com/gjtempleton)
diff --git a/moto/ecs/models.py b/moto/ecs/models.py
index e4258cee1..22835ecbb 100644
--- a/moto/ecs/models.py
+++ b/moto/ecs/models.py
@@ -652,6 +652,7 @@ class EC2ContainerServiceBackend(BaseBackend):
             '/')[-1]
         self.container_instances[cluster_name][
             container_instance_id] = container_instance
+        self.clusters[cluster_name].registered_container_instances_count += 1
         return container_instance
 
     def list_container_instances(self, cluster_str):
@@ -715,8 +716,10 @@ class EC2ContainerServiceBackend(BaseBackend):
                         resource["stringSetValue"].remove(str(port))
                     else:
                         resource["stringSetValue"].append(str(port))
+        container_instance.runningTaskCount += resource_multiplier * 1
 
     def deregister_container_instance(self, cluster_str, container_instance_str, force):
+        failures = []
         cluster_name = cluster_str.split('/')[-1]
         if cluster_name not in self.clusters:
             raise Exception("{0} is not a cluster".format(cluster_name))
@@ -724,18 +727,18 @@ class EC2ContainerServiceBackend(BaseBackend):
         container_instance = self.container_instances[cluster_name].get(container_instance_id)
         if container_instance is None:
             raise Exception("{0} is not a container id in the cluster")
-        if not force and container_instance.running_tasks_count > 0:
+        if not force and container_instance.runningTaskCount > 0:
             raise Exception("Found running tasks on the instance.")
         # Currently assume that people might want to do something based around deregistered instances
         # with tasks left running on them - but nothing if deregistration is forced or no tasks were
         # running already
-        elif force and container_instance.running_tasks_count > 0:
+        elif force and container_instance.runningTaskCount > 0:
             if not self.container_instances.get('orphaned'):
                 self.container_instances['orphaned'] = {}
             self.container_instances['orphaned'][container_instance_id] = container_instance
         del(self.container_instances[cluster_name][container_instance_id])
         self._respond_to_cluster_state_update(cluster_str)
-        pass
+        return container_instance, failures
 
     def _respond_to_cluster_state_update(self, cluster_str):
         cluster_name = cluster_str.split('/')[-1]
diff --git a/moto/ecs/responses.py b/moto/ecs/responses.py
index 6c8fd8e2d..50d9e3cd4 100644
--- a/moto/ecs/responses.py
+++ b/moto/ecs/responses.py
@@ -204,10 +204,12 @@ class EC2ContainerServiceResponse(BaseResponse):
         })
 
     def deregister_container_instance(self):
-        cluster_str = self._get_param('cluster', 'default'),
+        cluster_str = self._get_param('cluster')
+        if not cluster_str:
+            cluster_str = 'default'
         container_instance_str = self._get_param('containerInstance')
-        force = self._get_param('force', False)
-        container_instance = self.ecs_backend.deregister_container_instance(
+        force = self._get_param('force')
+        container_instance, failures = self.ecs_backend.deregister_container_instance(
             cluster_str, container_instance_str, force
         )
         return json.dumps({
diff --git a/tests/test_ecs/test_ecs_boto3.py b/tests/test_ecs/test_ecs_boto3.py
index e76be8cbe..7be782740 100644
--- a/tests/test_ecs/test_ecs_boto3.py
+++ b/tests/test_ecs/test_ecs_boto3.py
@@ -11,6 +11,7 @@ from uuid import UUID
 from moto import mock_cloudformation
 from moto import mock_ecs
 from moto import mock_ec2
+from nose.tools import assert_raises
 
 
 @mock_ecs
@@ -542,6 +543,92 @@ def test_register_container_instance():
         'dockerVersion'].should.equal('DockerVersion: 1.5.0')
 
 
+@mock_ec2
+@mock_ecs
+def test_deregister_container_instance():
+    ecs_client = boto3.client('ecs', region_name='us-east-1')
+    ec2 = boto3.resource('ec2', region_name='us-east-1')
+
+    test_cluster_name = 'test_ecs_cluster'
+
+    _ = ecs_client.create_cluster(
+        clusterName=test_cluster_name
+    )
+
+    test_instance = ec2.create_instances(
+        ImageId="ami-1234abcd",
+        MinCount=1,
+        MaxCount=1,
+    )[0]
+
+    instance_id_document = json.dumps(
+        ec2_utils.generate_instance_identity_document(test_instance)
+    )
+
+    response = ecs_client.register_container_instance(
+        cluster=test_cluster_name,
+        instanceIdentityDocument=instance_id_document
+    )
+    container_instance_id = response['containerInstance']['containerInstanceArn']
+    response = ecs_client.deregister_container_instance(
+        cluster=test_cluster_name,
+        containerInstance=container_instance_id
+    )
+    container_instances_response = ecs_client.list_container_instances(
+        cluster=test_cluster_name
+    )
+    len(container_instances_response['containerInstanceArns']).should.equal(0)
+
+    response = ecs_client.register_container_instance(
+        cluster=test_cluster_name,
+        instanceIdentityDocument=instance_id_document
+    )
+    container_instance_id = response['containerInstance']['containerInstanceArn']
+    _ = ecs_client.register_task_definition(
+        family='test_ecs_task',
+        containerDefinitions=[
+            {
+                'name': 'hello_world',
+                'image': 'docker/hello-world:latest',
+                'cpu': 1024,
+                'memory': 400,
+                'essential': True,
+                'environment': [{
+                    'name': 'AWS_ACCESS_KEY_ID',
+                    'value': 'SOME_ACCESS_KEY'
+                }],
+                'logConfiguration': {'logDriver': 'json-file'}
+            }
+        ]
+    )
+
+    response = ecs_client.start_task(
+        cluster='test_ecs_cluster',
+        taskDefinition='test_ecs_task',
+        overrides={},
+        containerInstances=[container_instance_id],
+        startedBy='moto'
+    )
+    with assert_raises(Exception) as e:
+        ecs_client.deregister_container_instance(
+            cluster=test_cluster_name,
+            containerInstance=container_instance_id
+        ).should.have.raised(Exception)
+    container_instances_response = ecs_client.list_container_instances(
+        cluster=test_cluster_name
+    )
+    len(container_instances_response['containerInstanceArns']).should.equal(1)
+    ecs_client.deregister_container_instance(
+        cluster=test_cluster_name,
+        containerInstance=container_instance_id,
+        force=True
+    )
+    container_instances_response = ecs_client.list_container_instances(
+        cluster=test_cluster_name
+    )
+    len(container_instances_response['containerInstanceArns']).should.equal(0)
+
+
 @mock_ec2
 @mock_ecs
 def test_list_container_instances():