diff --git a/src/Reflection/Reflector.php b/src/Reflection/Reflector.php
index 5bef5fb..1571ea1 100644
--- a/src/Reflection/Reflector.php
+++ b/src/Reflection/Reflector.php
@@ -188,7 +188,7 @@ private function doGetMessage(object $message, ?\Closure $presence = null): Mess
             if ($property->attributes->has(Field::class)) {
                 $field = $property->attributes->get(Field::class);
 
-                if ($presence($field, $value)) {
+                if (($property->default !== null && $property->default->value === null) || $presence($field, $value)) {
                     $descriptors[] = Protobuf\fieldOf(
                         $field->num,
                         $field->type->accept($this->valueVisitor)($value),
diff --git a/tests/Encoder/Internal/ReflectionEncoderTest.php b/tests/Encoder/Internal/ReflectionEncoderTest.php
index 65156e3..5098598 100644
--- a/tests/Encoder/Internal/ReflectionEncoderTest.php
+++ b/tests/Encoder/Internal/ReflectionEncoderTest.php
@@ -21,6 +21,16 @@ public function testEncode(): void
 
         self::assertSame('0a084a6f686e20446f651032', bin2hex($encoder->encode(new Request('John Doe', 50))));
     }
+
+    public function testEncodeOptionalScalarZeroValues(): void
+    {
+        $encoder = Builder::buildDefault();
+
+        self::assertSame(
+            '080012001800',
+            bin2hex($encoder->encode(new OptionalScalarRequest(0, '', false))),
+        );
+    }
 }
 
 final readonly class Request
@@ -32,3 +42,15 @@ public function __construct(
         public int $id,
     ) {}
 }
+
+final readonly class OptionalScalarRequest
+{
+    public function __construct(
+        #[Reflection\Field(1, Reflection\Int32T::T)]
+        public ?int $oneofIndex = null,
+        #[Reflection\Field(2, Reflection\StringT::T)]
+        public ?string $label = null,
+        #[Reflection\Field(3, Reflection\BoolT::T)]
+        public ?bool $enabled = null,
+    ) {}
+}