Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 34900)
+++ src/wp-includes/functions.php	(working copy)
@@ -2685,6 +2685,9 @@
 		$args = array( $data );
 	}
 
+	// Prepare the data for JSON serialization.
+	$data = _wp_json_prepare_data( $data );
+
 	$json = @call_user_func_array( 'json_encode', $args );
 
 	// If json_encode() was successful, no need to do more sanity checking.
@@ -2799,6 +2802,51 @@
 }
 
 /**
+ * Prepares response data to be serialized to JSON.
+ *
+ * This supports the JsonSerializable interface for PHP 5.2-5.3 as well.
+ *
+ * @ignore
+ * @since 4.4.0
+ * @access private
+ *
+ * @param mixed $data Native representation.
+ * @return bool|int|float|null|string|array Data ready for `json_encode()`.
+ */
+function _wp_json_prepare_data( $data ) {
+	if ( ! defined( 'WP_JSON_SERIALIZE_COMPATIBLE' ) || WP_JSON_SERIALIZE_COMPATIBLE === false ) {
+		return $data;
+	}
+
+	switch ( gettype( $data ) ) {
+		case 'boolean':
+		case 'integer':
+		case 'double':
+		case 'string':
+		case 'NULL':
+			// These values can be passed through.
+			return $data;
+
+		case 'array':
+			// Arrays must be mapped in case they also return objects.
+			return array_map( '_wp_json_prepare_data', $data );
+
+		case 'object':
+			if ( $data instanceof JsonSerializable ) {
+				$data = $data->jsonSerialize();
+			} else {
+				$data = get_object_vars( $data );
+			}
+
+			// Now, pass the array (or whatever was returned from jsonSerialize through).
+			return _wp_json_prepare_data( $data );
+
+		default:
+			return null;
+	}
+}
+
+/**
  * Send a JSON response back to an Ajax request.
  *
  * @since 3.5.0