Handle primitive types in serializerfields

Avoid crashes because the to_python helper expects data to be provided
as string.

phonenumber_field/serializerfields.py

@@ -9,7 +9,8 @@ class PhoneNumberField(serializers.CharField):
     default_error_messages = {"invalid": _("Enter a valid phone number.")}
 
     def to_internal_value(self, data):
-        phone_number = to_python(data)
+        str_value = super().to_internal_value(data)
+        phone_number = to_python(str_value)
         if phone_number and not phone_number.is_valid():
             raise ValidationError(self.error_messages["invalid"])
         return phone_number

tests/test_serializers.py

@@ -0,0 +1,13 @@
+from django.test import SimpleTestCase
+from rest_framework import serializers
+
+from phonenumber_field.serializerfields import PhoneNumberField
+
+
+class PhoneNumberFieldTest(SimpleTestCase):
+    def test_int(self):
+        class PhoneNumberSerializer(serializers.Serializer):
+            phone = PhoneNumberField()
+
+        s = PhoneNumberSerializer(data={"phone": 1})
+        self.assertIs(s.is_valid(), False)