diff --git a/src/BigInteger.php b/src/BigInteger.php
index 5bba58b..e173d77 100644
--- a/src/BigInteger.php
+++ b/src/BigInteger.php
@@ -225,9 +225,10 @@ public static function randomBits(int $numBits, ?callable $randomBytesGenerator
         }
 
         if ($randomBytesGenerator === null) {
-            $randomBytesGenerator = 'random_bytes';
+            $randomBytesGenerator = random_bytes(...);
         }
 
+        /** @var int<1, max> $byteLength */
         $byteLength = \intdiv($numBits - 1, 8) + 1;
 
         $extraBits = ($byteLength * 8 - $numBits);