diff --git a/.gitignore b/.gitignore
index 31a0e5d9..fd8ceeec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,2 @@
 vendor/
-build/
\ No newline at end of file
+build/
diff --git a/lib/HOTP.php b/lib/HOTP.php
index 3a4b3f27..08645641 100644
--- a/lib/HOTP.php
+++ b/lib/HOTP.php
@@ -29,8 +29,9 @@ public function verify($otp, $counter, $window = null)
         if (!is_integer($window)) {
             $window = 0;
         }
+        $window = abs($window);
 
-        for ($i=$counter; $i <= $counter+abs($window); $i++) {
+        for ($i=$counter; $i <= $counter+$window; $i++) {
             if ($otp === $this->at($i)) {
                 $this->updateCounter($i+1);
 
diff --git a/lib/TOTP.php b/lib/TOTP.php
index dc335e70..8621458b 100644
--- a/lib/TOTP.php
+++ b/lib/TOTP.php
@@ -32,8 +32,9 @@ public function verify($otp, $timestamp = null, $window = null)
         if (!is_integer($window)) {
             return $otp === $this->at($timestamp);
         }
+        $window = abs($window);
 
-        for ($i=-abs($window); $i <= abs($window); $i++) {
+        for ($i=-$window; $i <= $window; $i++) {
             if ($otp === $this->at($i*$this->getInterval()+$timestamp)) {
                 return true;
             }