diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4576953..9b97fd2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,9 +3,11 @@
 ## [Unreleased]
 
 - [#280](https://github.com/alecthomas/voluptuous/pull/280): Making
-`IsDir()`, `IsFile()` and `PathExists()` consistent between different python versions.
+  `IsDir()`, `IsFile()` and `PathExists()` consistent between different python versions.
 - [#279](https://github.com/alecthomas/voluptuous/pull/279):
   Treat Python 2 old-style classes like types when validating.
+- [324](https://github.com/alecthomas/voluptuous/pull/324):
+  Default values MUST now pass validation just as any regular value.
 
 ## [0.10.5]
 
diff --git a/voluptuous/schema_builder.py b/voluptuous/schema_builder.py
index dff3e55..88a0203 100644
--- a/voluptuous/schema_builder.py
+++ b/voluptuous/schema_builder.py
@@ -334,11 +334,25 @@ def _compile_mapping(self, schema, invalid_msg=None):
 
         def validate_mapping(path, iterable, out):
             required_keys = all_required_keys.copy()
-            # keeps track of all default keys that haven't been filled
-            default_keys = all_default_keys.copy()
+
+            # Build a map of all provided key-value pairs.
+            # The type(out) is used to retain ordering in case a ordered
+            # map type is provided as input.
+            key_value_map = type(out)()
+            for key, value in iterable:
+                key_value_map[key] = value
+
+            # Insert default values for non-existing keys.
+            for key in all_default_keys:
+                if not isinstance(key.default, Undefined) and \
+                   key.schema not in key_value_map:
+                    # A default value has been specified for this missing
+                    # key, insert it.
+                    key_value_map[key.schema] = key.default()
+
             error = None
             errors = []
-            for key, value in iterable:
+            for key, value in key_value_map.items():
                 key_path = path + [key]
                 remove_key = False
 
@@ -388,12 +402,10 @@ def validate_mapping(path, iterable, out):
                         required_keys.discard(skey)
                         break
 
-                    # Key and value okay, mark any Required() fields as found.
+                    # Key and value okay, mark as found in case it was
+                    # a Required() field.
                     required_keys.discard(skey)
 
-                    # No need for a default if it was filled
-                    default_keys.discard(skey)
-
                     break
                 else:
                     if remove_key:
@@ -405,13 +417,6 @@ def validate_mapping(path, iterable, out):
                         errors.append(er.Invalid('extra keys not allowed', key_path))
                         # else REMOVE_EXTRA: ignore the key so it's removed from output
 
-            # set defaults for any that can have defaults
-            for key in default_keys:
-                if not isinstance(key.default, Undefined):  # if the user provides a default with the node
-                    out[key.schema] = key.default()
-                    if key in required_keys:
-                        required_keys.discard(key)
-
             # for any required keys left that weren't found and don't have defaults:
             for key in required_keys:
                 msg = key.msg if hasattr(key, 'msg') and key.msg else 'required key not provided'