Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 42637)
+++ src/wp-includes/media.php	(working copy)
@@ -1205,7 +1205,7 @@
 	$sources = apply_filters( 'wp_calculate_image_srcset', $sources, $size_array, $image_src, $image_meta, $attachment_id );
 
 	// Only return a 'srcset' value if there is more than one source.
-	if ( ! $src_matched || count( $sources ) < 2 ) {
+	if ( ! $src_matched || ! is_array( $sources ) || count( $sources ) < 2 ) {
 		return false;
 	}