diff --git a/pyfarm/jobtypes/core/jobtype.py b/pyfarm/jobtypes/core/jobtype.py
index 6c665626..103fe6a3 100644
--- a/pyfarm/jobtypes/core/jobtype.py
+++ b/pyfarm/jobtypes/core/jobtype.py
@@ -531,6 +531,10 @@ def get_environment(self):
         """
         Constructs an environment dictionary that can be used
         when a process is spawned by a job type.
+
+        :raises TypeError:
+            Raised if ``jobtype_default_environment`` is defined
+            in the config and is not a dictionary.
         """
         environment = {}
         config_environment = config.get("jobtype_default_environment")
@@ -538,7 +542,12 @@ def get_environment(self):
         if config["jobtype_include_os_environ"]:
             environment.update(FROZEN_ENVIRONMENT)
 
-        if isinstance(config_environment, dict):
+        if config_environment is not None:
+            if not isinstance(config_environment, dict):
+                raise TypeError(
+                    "Expected the `jobtype_default_environment` configuration "
+                    "value to be a dictionary.")
+
             environment.update(config_environment)
 
         elif config_environment is not None:
@@ -550,20 +559,20 @@ def get_environment(self):
         # but it's possible to create an environment using the config which
         # contains non-strings.
         for key in environment:
+            value = environment.pop(key)
             if not isinstance(key, STRING_TYPES):
                 logger.warning(
                     "Environment key %r is not a string.  It will be converted "
                     "to a string.", key)
-
-                value = environment.pop(key)
                 key = str(key)
-                environment[key] = value
 
-            if not isinstance(environment[key], STRING_TYPES):
+            if not isinstance(value, STRING_TYPES):
                 logger.warning(
                     "Environment value for %r is not a string.  It will be "
                     "converted to a string.", key)
-                environment[key] = str(environment[key])
+                value = str(value)
+
+            environment[key] = value
 
         return environment
 
diff --git a/tests/test_jobtypes/test_core_jobtype.py b/tests/test_jobtypes/test_core_jobtype.py
index 51a278ab..57af601a 100644
--- a/tests/test_jobtypes/test_core_jobtype.py
+++ b/tests/test_jobtypes/test_core_jobtype.py
@@ -31,6 +31,7 @@
 from pyfarm.agent.sysinfo.user import is_administrator
 from pyfarm.jobtypes.core.internals import USER_GROUP_TYPES
 from pyfarm.jobtypes.core.jobtype import (
+    FROZEN_ENVIRONMENT, JobType, CommandData,
     JobType, CommandData, process_stdout, process_stderr, logger)
 from pyfarm.jobtypes.core.log import STDOUT, STDERR, logpool
 
@@ -219,6 +220,64 @@ def test_schema(self):
             JobType.load({})
 
 
+
+class TestJobTypeGetEnvironment(TestCase):
+    POP_CONFIG_KEYS = [
+        "jobtype_default_environment",
+        "jobtype_include_os_environ"
+    ]
+
+    def assertEnvironmentContains(self, target, contains):
+        for key, value in target.iteritems():
+            self.assertIn(key, contains)
+            self.assertEqual(target[key], contains[key])
+
+    def assertDoesNotEnvironmentContains(self, target, contains):
+        for key, value in target.iteritems():
+            self.assertNotIn(key, contains)
+
+    def test_includes_os_environ(self):
+        config["jobtype_include_os_environ"] = True
+        jobtype = JobType(fake_assignment())
+        self.assertEnvironmentContains(
+            FROZEN_ENVIRONMENT, jobtype.get_environment())
+
+    def test_does_not_include_os_environ(self):
+        config["jobtype_include_os_environ"] = False
+        jobtype = JobType(fake_assignment())
+        self.assertDoesNotEnvironmentContains(
+            FROZEN_ENVIRONMENT, jobtype.get_environment())
+
+    def test_include_config_environment(self):
+        config_env = config["jobtype_default_environment"] = {
+            "foo": "1", "bar": "2"
+        }
+        jobtype = JobType(fake_assignment())
+        self.assertEnvironmentContains(jobtype.get_environment(), config_env)
+
+    def test_bad_config_raises_type_error(self):
+        config["jobtype_default_environment"] = [""]
+        jobtype = JobType(fake_assignment())
+
+        with self.assertRaises(TypeError):
+            jobtype.get_environment()
+
+    def test_converts_non_string_key_to_string(self):
+        jobtype = JobType(fake_assignment())
+        config["jobtype_default_environment"] = {
+            1: "yes"
+        }
+        for key in jobtype.get_environment().keys():
+            self.assertIsInstance(key, str)
+
+    def test_converts_non_string_value_to_string(self):
+        jobtype = JobType(fake_assignment())
+        config["jobtype_default_environment"] = {
+            "yes": 1
+        }
+        for value in jobtype.get_environment().values():
+            self.assertIsInstance(value, str)
+
 class TestJobTypeFormatError(TestCase):
     def test_process_terminated(self):
         error = Failure(exc_value=Exception())