diff --git a/fkreplace/__init__.py b/fkreplace/__init__.py
index 4de821f..69e814d 100644
--- a/fkreplace/__init__.py
+++ b/fkreplace/__init__.py
@@ -1,5 +1,8 @@
 
 def merge(from_obj, to_obj):
+    if not isinstance(from_obj, type(to_obj)):
+        raise ValueError("both objects must be of the same type")
+
     for related in from_obj._meta.get_all_related_objects():
         accessor_name = related.get_accessor_name()
         varname = related.field.name
diff --git a/tests/tests.py b/tests/tests.py
index 648be1c..87c5701 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -8,6 +8,10 @@ class MergeTests(TestCase):
         self.b = Person.objects.create(name='bee')
         self.g = Group.objects.create(name='Team Awesome')
 
+    def test_same_class(self):
+        with self.assertRaises(ValueError):
+            merge(self.a, self.g)
+
     def test_fk_simple(self):
         Number.objects.create(person=self.a, number='555-1111')
         Number.objects.create(person=self.a, number='555-1112')
diff --git a/tox.ini b/tox.ini
index 74849f0..bdc8fc3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py27-django17, py34-django17, py27-django18, py34-django18, flake8
+envlist = py27-django18, py34-django18, flake8
 
 [testenv:flake8]
 deps = flake8