diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index 5105dfcb4..5fb99a42f 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -209,8 +209,10 @@ class Field(object):
         """
         Validate a simple representation and return the internal value.
 
-        The provided data may be `empty` if no representation was included.
-        May return `empty` if the field should not be included in the
+        The provided data may be `empty` if no representation was included
+        in the input.
+
+        May raise `SkipField` if the field should not be included in the
         validated data.
         """
         if data is empty:
@@ -223,6 +225,10 @@ class Field(object):
         return value
 
     def run_validators(self, value):
+        """
+        Test the given value against all the validators on the field,
+        and either raise a `ValidationError` or simply return.
+        """
         if value in (None, '', [], (), {}):
             return
 
@@ -753,8 +759,9 @@ class MultipleChoiceField(ChoiceField):
     }
 
     def to_internal_value(self, data):
-        if not hasattr(data, '__iter__'):
+        if isinstance(data, type('')) or not hasattr(data, '__iter__'):
             self.fail('not_a_list', input_type=type(data).__name__)
+
         return set([
             super(MultipleChoiceField, self).to_internal_value(item)
             for item in data
diff --git a/tests/test_fields.py b/tests/test_fields.py
index ae7f19193..e03ece544 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -5,22 +5,31 @@ import datetime
 import pytest
 
 
+def get_items(mapping_or_list_of_two_tuples):
+    # Tests accept either lists of two tuples, or dictionaries.
+    if isinstance(mapping_or_list_of_two_tuples, dict):
+        # {value: expected}
+        return mapping_or_list_of_two_tuples.items()
+    # [(value, expected), ...]
+    return mapping_or_list_of_two_tuples
+
+
 class ValidAndInvalidValues:
     """
-    Base class for testing valid and invalid field values.
+    Base class for testing valid and invalid input values.
     """
     def test_valid_values(self):
         """
         Ensure that valid values return the expected validated data.
         """
-        for input_value, expected_output in self.valid_mappings.items():
+        for input_value, expected_output in get_items(self.valid_mappings):
             assert self.field.run_validation(input_value) == expected_output
 
     def test_invalid_values(self):
         """
         Ensure that invalid values raise the expected validation error.
         """
-        for input_value, expected_failure in self.invalid_mappings.items():
+        for input_value, expected_failure in get_items(self.invalid_mappings):
             with pytest.raises(fields.ValidationError) as exc_info:
                 self.field.run_validation(input_value)
             assert exc_info.value.messages == expected_failure
@@ -189,14 +198,14 @@ class TestDecimalField(ValidAndInvalidValues):
         12.3: Decimal('12.3'),
         0.1: Decimal('0.1'),
     }
-    invalid_mappings = {
-        'abc': ["A valid number is required."],
-        Decimal('Nan'): ["A valid number is required."],
-        Decimal('Inf'): ["A valid number is required."],
-        '12.345': ["Ensure that there are no more than 3 digits in total."],
-        '0.01': ["Ensure that there are no more than 1 decimal places."],
-        123: ["Ensure that there are no more than 2 digits before the decimal point."]
-    }
+    invalid_mappings = (
+        ('abc', ["A valid number is required."]),
+        (Decimal('Nan'), ["A valid number is required."]),
+        (Decimal('Inf'), ["A valid number is required."]),
+        ('12.345', ["Ensure that there are no more than 3 digits in total."]),
+        ('0.01', ["Ensure that there are no more than 1 decimal places."]),
+        (123, ["Ensure that there are no more than 2 digits before the decimal point."])
+    )
     field = fields.DecimalField(max_digits=3, decimal_places=1)