diff --git a/src/Parser.php b/src/Parser.php
index f098b2d..b815f57 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -14,6 +14,9 @@
  */
 final class Parser implements ParserInterface
 {
+    // Bump this to invalidate all cached results (e.g. after changing the serialization format).
+    private const CACHE_VERSION = 1;
+
     /**
      * @var CacheManager|null
      */
@@ -171,12 +174,15 @@ public function parse(string $agent): ResultInterface
         if (! isset($this->runtime[$key])) {
             // In standalone mode, You can run the parser without a cache.
             if ($this->cache !== null) {
-                /** @var ResultInterface $result */
-                $result = $this->cache->remember(
+                // Cache the plain array to avoid __PHP_Incomplete_Class on deserialization.
+                /** @var array<string, mixed> $data */
+                $data = $this->cache->remember(
                     $key,
                     $this->cacheConfig()['interval'],
-                    fn () => $this->process($agent)
+                    fn () => $this->process($agent)->toArray()
                 );
+
+                $result = new Result($data);
             } else {
                 $result = $this->process($agent);
             }
@@ -192,7 +198,7 @@ public function parse(string $agent): ResultInterface
      */
     protected function makeHashKey(string $agent): string
     {
-        return $this->cacheConfig()['prefix'].hash('xxh128', $agent);
+        return $this->cacheConfig()['prefix'].self::CACHE_VERSION.'_'.hash('xxh128', $agent);
     }
 
     /**
diff --git a/tests/ParserTest.php b/tests/ParserTest.php
index f390bb0..72b14d2 100644
--- a/tests/ParserTest.php
+++ b/tests/ParserTest.php
@@ -177,4 +177,27 @@ public function test_detect_truncates_long_user_agent()
 
         $this->assertSame(10, strlen($result->userAgent()));
     }
+
+    /**
+     * Regression test for GitHub issue #16.
+     * Ensures cached results don't produce __PHP_Incomplete_Class errors.
+     *
+     * @covers ::parse()
+     */
+    public function test_cache_returns_result_interface_not_incomplete_class()
+    {
+        $parser = $this->getParser();
+        $agent = 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:149.0) Gecko/20100101 Firefox/149.0';
+
+        // First call populates the cache.
+        $first = $parser->parse($agent);
+        $this->assertInstanceOf(ResultInterface::class, $first);
+
+        // Create a new parser instance (no runtime cache) to force reading from the application cache.
+        $freshParser = $this->getParser();
+        $second = $freshParser->parse($agent);
+
+        $this->assertInstanceOf(ResultInterface::class, $second);
+        $this->assertSame($first->toArray(), $second->toArray());
+    }
 }