diff --git a/src/AWS/CRT/Internal/Encoding.php b/src/AWS/CRT/Internal/Encoding.php
index 75446fc..7c62559 100644
--- a/src/AWS/CRT/Internal/Encoding.php
+++ b/src/AWS/CRT/Internal/Encoding.php
@@ -24,7 +24,7 @@ final class Encoding {
     public static function decodeString($buffer) {
         $len = unpack("N", $buffer)[1];
         $buffer = substr($buffer, 4);
-        $str = unpack("a${len}", $buffer)[1];
+        $str = unpack("a{$len}", $buffer)[1];
         return [$len, $str];
     }