diff --git a/mapbox_location_field/models.py b/mapbox_location_field/models.py
index 30ce4a9..463186d 100644
--- a/mapbox_location_field/models.py
+++ b/mapbox_location_field/models.py
@@ -57,6 +57,9 @@ def to_python(self, value):
         return parse_location(value)
 
     def get_prep_value(self, value):
+        if value is None:
+            return value
+
         return "{},{}".format(value[0], value[1])
 
     def formfield(self, **kwargs):
diff --git a/mapbox_location_field/tests/test_models.py b/mapbox_location_field/tests/test_models.py
index 32abab9..d64d3e6 100644
--- a/mapbox_location_field/tests/test_models.py
+++ b/mapbox_location_field/tests/test_models.py
@@ -42,6 +42,7 @@ def test_to_python(self):
     def test_get_prep_value(self):
         instance = LocationField()
         self.assertEqual("1,5", instance.get_prep_value((1, 5)))
+        self.assertEqual(None, instance.get_prep_value(None))
 
     def test_form_field(self):
         instance = LocationField()
diff --git a/requirements.txt b/requirements.txt
deleted file mode 100644
index 1727f8d..0000000
--- a/requirements.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-Django==2.2.3
-pytz==2019.1
-sqlparse==0.3.0