diff --git a/flashcards/fields.py b/flashcards/fields.py
index 4569e74..2fe841b 100644
--- a/flashcards/fields.py
+++ b/flashcards/fields.py
@@ -1,4 +1,5 @@
 from django.db import models
+from validators import FlashcardMask, OverlapIntervalException
 
 
 class MaskField(models.Field):
@@ -37,16 +38,14 @@ class MaskField(models.Field):
         return ','.join(['-'.join(map(str, i)) for i in value])
 
     def to_python(self, value):
-        if value is None or isinstance(value, set):
+        if value is None:
             return value
-        return MaskField._parse_mask(value)
+        return sorted(list(FlashcardMask(value)))
 
     def get_prep_value(self, value):
         if value is None:
             return value
-        if not isinstance(value, set) or not all([isinstance(interval, tuple) for interval in value]):
-            raise ValueError("Invalid value for MaskField attribute")
-        return self.__class__._parse_mask(sorted(value))
+        return sorted(list(FlashcardMask(value)))
 
     def get_prep_lookup(self, lookup_type, value):
         raise TypeError("Lookup not supported for MaskField")
@@ -65,15 +64,9 @@ class MaskField(models.Field):
 
     @classmethod
     def _varchar_parse_mask(cls, value):
-        intervals = []
-        ranges = value.split(',')
-        for interval in ranges:
-            _range = interval.split('-')
-            if len(_range) != 2 or not all(map(unicode.isdigit, _range)):
-                raise ValueError("Invalid range format.")
-            intervals.append(tuple(_range))
-        return set([tuple(i) for i in cls._parse_mask(sorted(intervals))])
+        mask = [tuple(map(int, i.split('-'))) for i in value.split(',')]
+        return FlashcardMask(mask)
 
     @classmethod
     def _psql_parse_mask(cls, value):
-        return set([tuple(i) for i in cls._parse_mask(sorted(value))])
+        return FlashcardMask(value)
diff --git a/flashcards/tests/test_models.py b/flashcards/tests/test_models.py
index 6a64b06..5028cc9 100644
--- a/flashcards/tests/test_models.py
+++ b/flashcards/tests/test_models.py
@@ -2,6 +2,7 @@ from datetime import datetime
 
 from django.test import TestCase
 from flashcards.models import User, Section, Flashcard
+from flashcards.validators import OverlapIntervalException
 
 
 class RegistrationTests(TestCase):
@@ -68,5 +69,5 @@ class FlashcardTests(TestCase):
                                      previous=None,
                                      mask={(10,34), (0, 14)})
             self.fail()
-        except ValueError:
+        except OverlapIntervalException:
             self.assertTrue(True)
diff --git a/flashcards/validators.py b/flashcards/validators.py
new file mode 100644
index 0000000..81d8417
--- /dev/null
+++ b/flashcards/validators.py
@@ -0,0 +1,36 @@
+__author__ = 'rray'
+
+from collections import Iterable
+
+
+class FlashcardMask(set):
+    def __init__(self, *args, **kwargs):
+        super(FlashcardMask, self).__init__(*args, **kwargs)
+        self._iterable_check()
+        self._interval_check()
+        self._overlap_check()
+
+    def _iterable_check(self):
+        if not all([isinstance(i, Iterable) for i in self]):
+            raise TypeError("Interval not a valid iterable")
+
+    def _interval_check(self):
+        if not all([len(i) == 2 for i in self]):
+            raise TypeError("Intervals must have exactly 2 elements, begin and end")
+
+    def _overlap_check(self):
+        p_beg, p_end = -1, -1
+        for interval in sorted(self):
+            beg, end = map(int, interval)
+            if not (0 <= beg <= 255) or not (0 <= end <= 255) or not (beg <= end) or not (beg > p_end):
+                raise OverlapIntervalException((beg, end), "Invalid interval offsets in the mask")
+            p_beg, p_end = beg, end
+
+
+class OverlapIntervalException(Exception):
+    def __init__(self, interval, reason):
+        self.interval = interval
+        self.reason = reason
+
+    def __str__(self):
+        return repr(self.reason) + ': ' + repr(self.interval)
diff --git a/flashcards/views.py b/flashcards/views.py
index 355f3d0..b7c89f6 100644
--- a/flashcards/views.py
+++ b/flashcards/views.py
@@ -273,7 +273,7 @@ class FlashcardViewSet(GenericViewSet, CreateModelMixin, RetrieveModelMixin):
         :return: A 204 response upon success.
         """
         user = request.user
-        flashcard = Flashcard.objects.get(pk=pk)
+        flashcard = self.get_object()
         user_card, created = UserFlashcard.objects.get_or_create(user=user, flashcard=flashcard)
         user_card.save()
         return Response(status=HTTP_204_NO_CONTENT)