diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index 6caae9242..8e30e77a3 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -429,6 +429,11 @@ class ModelField(WritableField):
             "type": self.model_field.get_internal_type()
         }
 
+    def validate(self, value):
+        super(ModelField, self).validate(value)
+        if value is None and not self.model_field.null:
+            raise ValidationError(self.error_messages['invalid'])
+
 
 ##### Typed Fields #####
 
@@ -474,10 +479,7 @@ class CharField(WritableField):
             self.validators.append(validators.MaxLengthValidator(max_length))
 
     def from_native(self, value):
-        if isinstance(value, six.string_types):
-            return value
-
-        if value is None:
+        if value in validators.EMPTY_VALUES:
             return ''
 
         return smart_text(value)