diff --git a/lib/DateTimeParser.php b/lib/DateTimeParser.php
index 1c253437..c5dbac97 100644
--- a/lib/DateTimeParser.php
+++ b/lib/DateTimeParser.php
@@ -104,7 +104,7 @@ public static function parseDuration($duration, $asString = false)
         if (!$asString) {
             $invert = false;
 
-            if ('-' === $matches['plusminus']) {
+            if (isset($matches['plusminus']) && '-' === $matches['plusminus']) {
                 $invert = true;
             }