diff --git a/django_pandas/tests/models.py b/django_pandas/tests/models.py
index 6efbb0a..7ebd0da 100644
--- a/django_pandas/tests/models.py
+++ b/django_pandas/tests/models.py
@@ -116,7 +116,7 @@ def __unicode__(self):
 
 class TradeLog(models.Model):
     trader = models.ForeignKey(Trader)
-    symbol = models.ForeignKey(Security)
+    symbol = models.ForeignKey(Security, null=True)
     log_datetime = models.DateTimeField()
     price = models.FloatField()
     volume = models.IntegerField()
diff --git a/django_pandas/tests/test_io.py b/django_pandas/tests/test_io.py
index 4281704..d722043 100644
--- a/django_pandas/tests/test_io.py
+++ b/django_pandas/tests/test_io.py
@@ -70,10 +70,10 @@ def setUp(self):
         fish = Trader.objects.create(name="Fred Fish")
         abc = Security.objects.create(symbol='ABC', isin='999901')
         zyz = Security.objects.create(symbol='ZYZ', isin='999907')
-        TradeLog.objects.create(trader=bob, symbol=abc,
+        TradeLog.objects.create(trader=bob, symbol=None,
                                 log_datetime='2013-01-01T09:30:00',
                                 price=30, volume=300)
-        TradeLog.objects.create(trader=bob, symbol=abc,
+        TradeLog.objects.create(trader=bob, symbol=None,
                                 log_datetime='2013-01-01T10:00:00',
                                 price=30, volume=300)
         TradeLog.objects.create(trader=bob, symbol=abc,
diff --git a/django_pandas/utils.py b/django_pandas/utils.py
index c725792..2a87efa 100644
--- a/django_pandas/utils.py
+++ b/django_pandas/utils.py
@@ -1,5 +1,6 @@
 # coding: utf-8
 
+from math import isnan
 from django.core.cache import cache
 from django.utils.encoding import force_text
 
@@ -31,7 +32,7 @@ def replace_pk(model):
     base_cache_key = get_base_cache_key(model)
 
     def inner(pk_list):
-        cache_keys = [None if pk is None else base_cache_key % pk
+        cache_keys = [None if isnan(pk) else base_cache_key % pk
                       for pk in pk_list]
         out_dict = cache.get_many(frozenset(cache_keys))
         try:
@@ -41,7 +42,7 @@ def inner(pk_list):
             out_dict = {
                 base_cache_key % obj.pk: force_text(obj)
                 for obj in model.objects.filter(pk__in={pk for pk in pk_list
-                                                        if pk is not None})}
+                                                        if not isnan(pk)})}
             cache.set_many(out_dict)
             out_list = list(map(out_dict.get, cache_keys))
         return out_list