diff --git a/src/wp-includes/compat.php b/src/wp-includes/compat.php
index 531c76fe1b2b3..3447d272a469e 100644
--- a/src/wp-includes/compat.php
+++ b/src/wp-includes/compat.php
@@ -418,6 +418,9 @@ function array_key_first( array $array ) { // phpcs:ignore Universal.NamingConve
 		foreach ( $array as $key => $value ) {
 			return $key;
 		}
+
+		// Return null for an empty array.
+		return null;
 	}
 }
 
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index c4db4871df644..8852519ec45c9 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1649,6 +1649,8 @@ protected function handle_terms( $post_id, $request ) {
 				return $result;
 			}
 		}
+
+		return null;
 	}
 
 	/**