diff --git a/src/Once.php b/src/Once.php
index 1c1b24e..44f54a8 100644
--- a/src/Once.php
+++ b/src/Once.php
@@ -20,15 +20,17 @@ final class Once
     private ?Future $future = null;
 
     /**
-     * @var ?T
+     * @var T
      */
-    private mixed $value = null;
+    private mixed $value;
 
     /**
      * @var \Closure(T): bool
      */
     private readonly mixed $isAlive;
 
+    private bool $isResolved = false;
+
     /**
      * @param \Closure(): T $function
      * @param ?\Closure(T): bool $isAlive
@@ -45,16 +47,22 @@ public function __construct(
      */
     public function await(?Cancellation $cancellation = null): mixed
     {
-        if ($this->value !== null && ($this->isAlive)($this->value)) {
+        if ($this->isResolved && ($this->isAlive)($this->value)) {
             return $this->value;
         }
 
+        $this->isResolved = false;
+
         $this->future ??= async($this->function);
 
         try {
-            return $this->value = $this->future->await($cancellation);
+            $this->value = $this->future->await($cancellation);
         } finally {
             $this->future = null;
         }
+
+        $this->isResolved = true;
+
+        return $this->value;
     }
 }
diff --git a/tests/.gitignore b/tests/.gitignore
deleted file mode 100644
index e69de29..0000000
diff --git a/tests/OnceTest.php b/tests/OnceTest.php
new file mode 100644
index 0000000..62d44a7
--- /dev/null
+++ b/tests/OnceTest.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Thesis\Sync;
+
+use Amp\DeferredFuture;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use function Amp\async;
+
+#[CoversClass(Once::class)]
+final class OnceTest extends TestCase
+{
+    public function testReturnsOnceSameValue(): void
+    {
+        /** @var DeferredFuture<null> */
+        $deferred = new DeferredFuture();
+        $once = new Once(static function () use ($deferred): string {
+            $deferred->getFuture()->await();
+
+            return random_bytes(8);
+        });
+        $future1 = async(static fn() => $once->await());
+        $future2 = async(static fn() => $once->await());
+
+        async(static function () use ($deferred, $future1, $future2): void {
+            self::assertFalse($future1->isComplete());
+            self::assertFalse($future2->isComplete());
+
+            $deferred->complete();
+
+            self::assertSame($future1->await(), $future2->await());
+        })->await();
+    }
+
+    public function testWorksWithNull(): void
+    {
+        /** @var DeferredFuture<null> */
+        $deferred = new DeferredFuture();
+        $once = new Once(static fn(): null => $deferred->getFuture()->await());
+        $future = async(static fn() => $once->await());
+
+        async(static function () use ($deferred, $future): void {
+            self::assertFalse($future->isComplete());
+
+            $deferred->complete();
+
+            self::assertNull($future->await());
+        })->await();
+    }
+
+    public function testIsAlive(): void
+    {
+        $once = new Once(
+            static function (): int {
+                /** @var int */
+                static $i = 0;
+
+                return ++$i;
+            },
+            static fn(int $i): bool => $i > 1,
+        );
+
+        self::assertSame(1, $once->await());
+        self::assertSame(2, $once->await());
+        self::assertSame(2, $once->await());
+        self::assertSame(2, $once->await());
+    }
+}