diff --git a/wp-includes/rest-api.php b/wp-includes/rest-api.php
index 01a1a9a50c..47b922bcd8 100644
--- a/wp-includes/rest-api.php
+++ b/wp-includes/rest-api.php
@@ -103,6 +103,18 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 				'5.5.0'
 			);
 		}
+
+		if ( count( array_filter( $arg_group['args'], 'is_array' ) ) !== count( $arg_group['args'] ) ) {
+			_doing_it_wrong(
+				__FUNCTION__,
+				sprintf(
+				/* translators: %s: The REST API route being registered. */
+					__( 'REST API $args should be an array of arrays. Non-array value detected for %s.' ),
+					'<code>' . $clean_namespace . '/' . trim( $route, '/' ) . '</code>'
+				),
+				'6.1.0'
+			);
+		}
 	}
 
 	$full_route = '/' . $clean_namespace . '/' . trim( $route, '/' );
diff --git a/wp-includes/rest-api/class-wp-rest-server.php b/wp-includes/rest-api/class-wp-rest-server.php
index ddb3f6c01b..6327d5a539 100644
--- a/wp-includes/rest-api/class-wp-rest-server.php
+++ b/wp-includes/rest-api/class-wp-rest-server.php
@@ -1513,6 +1513,11 @@ class WP_REST_Server {
 				$endpoint_data['args'] = array();
 
 				foreach ( $callback['args'] as $key => $opts ) {
+					if ( is_string( $opts ) ) {
+						$opts = array( $opts => 0 );
+					} elseif ( ! is_array( $opts ) ) {
+						$opts = array();
+					}
 					$arg_data             = array_intersect_key( $opts, $allowed_schema_keywords );
 					$arg_data['required'] = ! empty( $opts['required'] );
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 1a3576cff5..74f340a574 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-beta2-54338';
+$wp_version = '6.1-beta2-54339';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.