Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 45452)
+++ src/wp-includes/meta.php	(working copy)
@@ -207,7 +207,7 @@
 	// Compare existing value to new value if no prev value given and the key exists only once.
 	if ( empty( $prev_value ) ) {
 		$old_value = get_metadata( $meta_type, $object_id, $meta_key );
-		if ( count( $old_value ) == 1 ) {
+		if ( (is_array($old_value) || is_object($old_value)) && 1 === count( $old_value ) ) {
 			if ( $old_value[0] === $meta_value ) {
 				return false;
 			}