diff --git a/src/Client.php b/src/Client.php
index 4520c25..7d642b4 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -195,7 +195,7 @@ public function endMemoryProfile(string $key, float $sampleRate = 1.0, array $ta
      * @param float $sampleRate
      * @param array $tags
      */
-    public function memory(string $key, int $memory = null, float $sampleRate = 1.0, array $tags = []): void
+    public function memory(string $key, ?int $memory = null, float $sampleRate = 1.0, array $tags = []): void
     {
         if ($memory === null) {
             $memory = memory_get_peak_usage();