diff --git a/TextDiff.php b/TextDiff.php
index 6590de7..afe2009 100644
--- a/TextDiff.php
+++ b/TextDiff.php
@@ -149,7 +149,7 @@ private function getWordDiff($old_array, $new_array)
                     ++$count;
                 } else {
                     $before = $count -1;
-                    if(array_key_exists($before, $words) && !array_key_exists('source', $words[$before])) {
+                    if(array_key_exists($before, $words) && is_array($words[$before]) && !array_key_exists('source', $words[$before])) {
                         $words[$before] .= $val;
                     } else {
                         $words[] = $val;