diff --git a/src/Xdiff.php b/src/Xdiff.php
index efc4f03..6825391 100644
--- a/src/Xdiff.php
+++ b/src/Xdiff.php
@@ -467,7 +467,7 @@ class Xdiff
         if (! is_string($side2)) {
             throw new \Exception("XDiff : side2 parameter not a string");
         }
-        if (trim($symbol) === "") {
+        if ($symbol !== null && trim($symbol) === "") {
             $symbol = null;
         }
         $side1 = mb_substr(rtrim($side1), 0, $this->s2sWidth - 1);