diff --git a/src/Once.php b/src/Once.php
index 44f54a8..3f53f13 100644
--- a/src/Once.php
+++ b/src/Once.php
@@ -24,11 +24,6 @@ final class Once
      */
     private mixed $value;
 
-    /**
-     * @var \Closure(T): bool
-     */
-    private readonly mixed $isAlive;
-
     private bool $isResolved = false;
 
     /**
@@ -36,18 +31,16 @@ final class Once
      * @param ?\Closure(T): bool $isAlive
      */
     public function __construct(
-        private readonly \Closure $function,
-        ?\Closure $isAlive = null,
-    ) {
-        $this->isAlive = $isAlive ?? static fn(): true => true;
-    }
+        private \Closure $function,
+        private readonly ?\Closure $isAlive = null,
+    ) {}
 
     /**
      * @return T
      */
     public function await(?Cancellation $cancellation = null): mixed
     {
-        if ($this->isResolved && ($this->isAlive)($this->value)) {
+        if ($this->isResolved && ($this->isAlive === null || ($this->isAlive)($this->value))) {
             return $this->value;
         }
 
@@ -63,6 +56,10 @@ public function await(?Cancellation $cancellation = null): mixed
 
         $this->isResolved = true;
 
+        if ($this->isAlive === null) {
+            $this->function = static fn() => throw new \LogicException('Function has been freed from memory');
+        }
+
         return $this->value;
     }
 }
diff --git a/tests/OnceTest.php b/tests/OnceTest.php
index 62d44a7..47190d4 100644
--- a/tests/OnceTest.php
+++ b/tests/OnceTest.php
@@ -67,4 +67,18 @@ static function (): int {
         self::assertSame(2, $once->await());
         self::assertSame(2, $once->await());
     }
+
+    public function testFunctionIsFreedIfIsAliveIsNull(): void
+    {
+        $value = new \stdClass();
+        $weakValue = \WeakReference::create($value);
+        $once = new Once(static fn() => $value::class);
+        unset($value);
+
+        self::assertNotNull($weakValue->get());
+
+        $once->await();
+
+        self::assertNull($weakValue->get());
+    }
 }