diff --git a/checks.d/docker.py b/checks.d/docker.py
index 44aed36804..3d74d44ce0 100644
--- a/checks.d/docker.py
+++ b/checks.d/docker.py
@@ -109,8 +109,8 @@ def unix_open(self, req):
 class Docker(AgentCheck):
     """Collect metrics and events from Docker API and cgroups"""
 
-    def __init__(self, name, init_config, agentConfig):
-        AgentCheck.__init__(self, name, init_config, agentConfig)
+    def __init__(self, name, init_config, agentConfig, instances=None):
+        AgentCheck.__init__(self, name, init_config, agentConfig, instances)
 
         # Initialize a HTTP opener with Unix socket support
         socket_timeout = int(init_config.get('socket_timeout', 0)) or DEFAULT_SOCKET_TIMEOUT
@@ -415,11 +415,13 @@ def _get_cgroup_file(self, cgroup, container_id, filename):
     def _find_cgroup(self, hierarchy, docker_root):
         """Finds the mount point for a specified cgroup hierarchy. Works with
         old style and new style mounts."""
+        fp = None
         try:
             fp = open(os.path.join(docker_root, "/proc/mounts"))
             mounts = map(lambda x: x.split(), fp.read().splitlines())
         finally:
-            fp.close()
+            if fp is not None:
+                fp.close()
         cgroup_mounts = filter(lambda x: x[2] == "cgroup", mounts)
         if len(cgroup_mounts) == 0:
             raise Exception("Can't find mounted cgroups. If you run the Agent inside a container,"
diff --git a/tests/common.py b/tests/common.py
index 41e8ed1310..cdc78fc4e8 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -11,6 +11,26 @@
 from config import get_checksd_path
 from util import get_os, get_hostname
 
+def get_check_class(name):
+    checksd_path = get_checksd_path(get_os())
+    if checksd_path not in sys.path:
+        sys.path.append(checksd_path)
+
+    check_module = __import__(name)
+    check_class = None
+    classes = inspect.getmembers(check_module, inspect.isclass)
+    for _, clsmember in classes:
+        if clsmember == AgentCheck:
+            continue
+        if issubclass(clsmember, AgentCheck):
+            check_class = clsmember
+            if AgentCheck in clsmember.__bases__:
+                continue
+            else:
+                break
+
+    return check_class
+
 def load_check(name, config, agentConfig):
     checksd_path = get_checksd_path(get_os())
     if checksd_path not in sys.path:
diff --git a/tests/test_docker.py b/tests/test_docker.py
new file mode 100644
index 0000000000..3d1d927b77
--- /dev/null
+++ b/tests/test_docker.py
@@ -0,0 +1,77 @@
+import unittest
+
+from mock import patch
+
+from tests.common import get_check_class
+
+def _mocked_find_cgroup(*args, **kwargs):
+    return
+
+class DockerCheckTest(unittest.TestCase):
+    def test_tag_exclude_all(self):
+        """ exclude all, except ubuntu and debian. """
+        instance = {
+            'include': [
+                'docker_image:ubuntu',
+                'docker_image:debian',
+            ],
+            'exclude': ['.*'],
+        }
+
+        klass = get_check_class('docker')
+        # NO-OP but loads the check
+        with patch.object(klass, '_find_cgroup', _mocked_find_cgroup):
+            check = klass('docker', {}, {})
+
+        check._prepare_filters(instance)
+        self.assertEquals(len(instance['exclude_patterns']), 1)
+        self.assertEquals(len(instance['include_patterns']), 2)
+
+        truth_table_exclusion = {
+            'some_tag': True,
+            'debian:ubuntu': True,
+            'docker_image:centos': True,
+            'docker_image:ubuntu': False,
+            'docker_image:debian': False,
+        }
+
+        for tag, val in truth_table_exclusion.iteritems():
+            self.assertEquals(
+                check._is_container_excluded(instance, [tag]),
+                val,
+                "{0} expected {1} but is not".format(tag, val)
+            )
+
+    def test_tag_include_all(self):
+        """ exclude all, except ubuntu and debian. """
+        instance = {
+            'include': [],
+            'exclude': [
+                'docker_image:ubuntu',
+                'docker_image:debian',
+            ],
+        }
+
+        klass = get_check_class('docker')
+        # NO-OP but loads the check
+        with patch.object(klass, '_find_cgroup', _mocked_find_cgroup):
+            check = klass('docker', {}, {})
+
+        check._prepare_filters(instance)
+        self.assertEquals(len(instance['exclude_patterns']), 2)
+        self.assertEquals(len(instance['include_patterns']), 0)
+
+        truth_table_exclusion = {
+            'some_tag': False,
+            'debian:ubuntu': False,
+            'docker_image:centos': False,
+            'docker_image:ubuntu': True,
+            'docker_image:debian': True,
+        }
+
+        for tag, val in truth_table_exclusion.iteritems():
+            self.assertEquals(
+                check._is_container_excluded(instance, [tag]),
+                val,
+                "{0} expected {1} but is not".format(tag, val)
+            )