diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index cbc8586c4..1e7981ecc 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -990,8 +990,7 @@ class DecimalField(Field):
         'max_digits': _('Ensure that there are no more than {max_digits} digits in total.'),
         'max_decimal_places': _('Ensure that there are no more than {max_decimal_places} decimal places.'),
         'max_whole_digits': _('Ensure that there are no more than {max_whole_digits} digits before the decimal point.'),
-        'max_string_length': _('String value too large.'),
-        'invalid_rounding': _('Invalid rounding option {rounding}. Valid values for rounding are: {valid_roundings}')
+        'max_string_length': _('String value too large.')
     }
     MAX_STRING_LENGTH = 1000  # Guard against malicious string inputs.
 
@@ -1028,9 +1027,10 @@ class DecimalField(Field):
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
-        valid_roundings = [v for k, v in vars(decimal).items() if k.startswith('ROUND_')]
-        if rounding is not None and rounding not in valid_roundings:
-            self.fail('invalid_rounding', rounding=rounding, valid_roundings=valid_roundings)
+        if rounding is not None:
+            valid_roundings = [v for k, v in vars(decimal).items() if k.startswith('ROUND_')]
+            assert rounding in valid_roundings, \
+                'Invalid rounding option %s. Valid values for rounding are: %s' % (rounding, valid_roundings)
         self.rounding = rounding
 
     def to_internal_value(self, data):