diff --git a/test_project/one_to_one/fixtures/one_to_one.json b/test_project/one_to_one/fixtures/one_to_one.json
index b53f364..4e627f5 100644
--- a/test_project/one_to_one/fixtures/one_to_one.json
+++ b/test_project/one_to_one/fixtures/one_to_one.json
@@ -54,5 +54,12 @@
         "name": "Steve"
     },
     "pk": 3
+},
+{
+    "model": "one_to_one.waiter",
+    "fields": {
+        "name": "Michal"
+    },
+    "pk": 4
 }
 ]
diff --git a/test_project/one_to_one/models.py b/test_project/one_to_one/models.py
index a2e4377..e0eb7f6 100644
--- a/test_project/one_to_one/models.py
+++ b/test_project/one_to_one/models.py
@@ -26,7 +26,7 @@ def __str__(self):
 
 @six.python_2_unicode_compatible
 class Waiter(models.Model):
-    restaurant = models.ForeignKey(Restaurant)
+    restaurant = models.ForeignKey(Restaurant, null=True)
     name = models.CharField(max_length=50)
 
     def __str__(self):
diff --git a/tests/filtersets/test_base.py b/tests/filtersets/test_base.py
index a41b118..397cfe3 100644
--- a/tests/filtersets/test_base.py
+++ b/tests/filtersets/test_base.py
@@ -231,3 +231,17 @@ def _test(fs, data, qs, expected, count):
              .exclude(name__icontains='jon')),
             1
         )
+        _test(
+            WaiterFilterSet,
+            'restaurant__isnull=True',
+            Waiter.objects.all(),
+            Waiter.objects.filter(restaurant__isnull=True),
+            1
+        )
+        _test(
+            WaiterFilterSet,
+            'restaurant__isnull=False',
+            Waiter.objects.all(),
+            Waiter.objects.filter(restaurant__isnull=False),
+            3
+        )
diff --git a/url_filter/filters.py b/url_filter/filters.py
index b538594..7289a64 100644
--- a/url_filter/filters.py
+++ b/url_filter/filters.py
@@ -17,7 +17,7 @@
 }
 
 LOOKUP_FIELD_OVERWRITES = {
-    'isnull': forms.BooleanField(),
+    'isnull': forms.BooleanField(required=False),
     'second': forms.IntegerField(min_value=0, max_value=59),
     'minute': forms.IntegerField(min_value=0, max_value=59),
     'hour': forms.IntegerField(min_value=0, max_value=23),
diff --git a/url_filter/filtersets/base.py b/url_filter/filtersets/base.py
index 96d13e1..4a0f716 100644
--- a/url_filter/filtersets/base.py
+++ b/url_filter/filtersets/base.py
@@ -20,6 +20,10 @@
 
 __all__ = ['FilterSet', 'FilterSetOptions', 'StrictMode']
 
+FK_FIELD_FILTERS = {
+    'isnull',
+}
+
 
 class StrictMode(enum.Enum):
     """
@@ -154,6 +158,7 @@ def _init(self, data=None, queryset=None, context=None,
         self.queryset = queryset
         self.context = context or {}
         self.strict_mode = strict_mode
+        self._given_lookups = None
 
     def repr(self, prefix=''):
         header = '{name}()'.format(name=self.__class__.__name__)
@@ -347,7 +352,10 @@ def get_spec(self, config):
             value = LookupConfig(config.key, config.data)
 
         if name not in self.filters:
-            raise SkipFilter
+            if name in FK_FIELD_FILTERS:
+                return super(FilterSet, self).get_spec(config)
+            else:
+                raise SkipFilter
 
         return self.filters[name].get_spec(value)