diff --git a/lib/avro/datum.php b/lib/avro/datum.php
index 1a17a06..f6dbd69 100644
--- a/lib/avro/datum.php
+++ b/lib/avro/datum.php
@@ -76,7 +76,7 @@ class AvroIODatumWriter
    * Schema used by this instance to write Avro data.
    * @var AvroSchema
    */
-  private $writers_schema;
+  public $writers_schema;
 
   /**
    * @param AvroSchema $writers_schema
diff --git a/lib/avro/schema.php b/lib/avro/schema.php
index a8853d4..9401e11 100644
--- a/lib/avro/schema.php
+++ b/lib/avro/schema.php
@@ -459,9 +459,11 @@ public static function is_valid_datum($expected_schema, $datum)
       case self::REQUEST_SCHEMA:
         if (is_array($datum))
         {
-          foreach ($expected_schema->fields() as $field)
-            if (!array_key_exists($field->name(), $datum) || !self::is_valid_datum($field->type(), $datum[$field->name()]))
-              return false;
+          foreach ($expected_schema->fields() as $field) {
+              $value = isset($datum[$field->name()]) ? $datum[$field->name()] : $field->default_value();
+              if (!self::is_valid_datum($field->type(), $value))
+                  return false;
+          }
           return true;
         }
         return false;