diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 9194c78df7..669026ecf0 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -1243,8 +1243,10 @@ function register_post_type( $post_type, $args = array() ) {
 	$post_type = sanitize_key( $post_type );
 	$args->name = $post_type;
 
-	if ( strlen( $post_type ) > 20 )
+	if ( strlen( $post_type ) > 20 ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Post types cannot exceed 20 characters in length' ), '4.0' );
 		return new WP_Error( 'post_type_too_long', __( 'Post types cannot exceed 20 characters in length' ) );
+	}
 
 	// If not set, default to the setting for public.
 	if ( null === $args->publicly_queryable )
diff --git a/src/wp-includes/taxonomy.php b/src/wp-includes/taxonomy.php
index 1f91f27def..426e8decce 100644
--- a/src/wp-includes/taxonomy.php
+++ b/src/wp-includes/taxonomy.php
@@ -346,8 +346,10 @@ function register_taxonomy( $taxonomy, $object_type, $args = array() ) {
 	);
 	$args = wp_parse_args( $args, $defaults );
 
-	if ( strlen( $taxonomy ) > 32 )
+	if ( strlen( $taxonomy ) > 32 ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Taxonomies cannot exceed 32 characters in length' ), '4.0' );
 		return new WP_Error( 'taxonomy_too_long', __( 'Taxonomies cannot exceed 32 characters in length' ) );
+	}
 
 	if ( false !== $args['query_var'] && ! empty( $wp ) ) {
 		if ( true === $args['query_var'] )