diff --git a/phpseclib/Crypt/Hash.php b/phpseclib/Crypt/Hash.php
index 2c248eb1..17f17d5e 100644
--- a/phpseclib/Crypt/Hash.php
+++ b/phpseclib/Crypt/Hash.php
@@ -569,10 +569,10 @@ class Hash
         // For the last chunk: pad to 32-byte boundary, endian-adjust,
         // NH hash and add bit-length.  Concatenate the result to Y.
         //
-        $length = strlen($m[$i]);
+        $length = count($m) ? strlen($m[$i]) : 0;
         $pad = 32 - ($length % 32);
         $pad = max(32, $length + $pad % 32);
-        $m[$i] = str_pad($m[$i], $pad, "\0"); // zeropad
+        $m[$i] = str_pad(isset($m[$i]) ? $m[$i] : '', $pad, "\0"); // zeropad
         $m[$i] = pack('N*', ...unpack('V*', $m[$i])); // ENDIAN-SWAP
 
         $y .= static::nh($k, $m[$i], new BigInteger($length * 8));