diff --git a/ext/reflection/php_reflection.c b/ext/reflection/php_reflection.c
index f5e463699b1b5..724de0a491240 100644
--- a/ext/reflection/php_reflection.c
+++ b/ext/reflection/php_reflection.c
@@ -4128,7 +4128,7 @@ ZEND_METHOD(ReflectionClass, getStaticPropertyValue)
 	prop = zend_std_get_static_property(ce, name, BP_VAR_IS);
 	EG(fake_scope) = old_scope;
 
-	if (prop) {
+	if (prop && !Z_ISUNDEF_P(prop)) {
 		RETURN_COPY_DEREF(prop);
 	}
 
@@ -4136,8 +4136,13 @@ ZEND_METHOD(ReflectionClass, getStaticPropertyValue)
 		RETURN_COPY(def_value);
 	}
 
-	zend_throw_exception_ex(reflection_exception_ptr, 0,
-		"Property %s::$%s does not exist", ZSTR_VAL(ce->name), ZSTR_VAL(name));
+	if (prop) {
+		zend_throw_error(NULL,
+			"Typed property %s::$%s must not be accessed before initialization", ZSTR_VAL(ce->name), ZSTR_VAL(name));
+	} else {
+		zend_throw_exception_ex(reflection_exception_ptr, 0,
+			"Property %s::$%s does not exist", ZSTR_VAL(ce->name), ZSTR_VAL(name));
+	}
 }
 /* }}} */
 
diff --git a/ext/reflection/tests/gh12856.phpt b/ext/reflection/tests/gh12856.phpt
new file mode 100644
index 0000000000000..519a222ab0ed7
--- /dev/null
+++ b/ext/reflection/tests/gh12856.phpt
@@ -0,0 +1,27 @@
+--TEST--
+GH-12856 (ReflectionClass::getStaticPropertyValue() returns UNDEF zval for uninitialized typed properties)
+--FILE--
+<?php
+
+class Bug {
+    private static $untyped;
+    private static int $typed1;
+    private static int $typed2 = 3;
+}
+
+$rc = new ReflectionClass(Bug::class);
+var_dump($rc->getStaticPropertyValue('untyped'));
+try {
+    var_dump($rc->getStaticPropertyValue('typed1'));
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+var_dump($rc->getStaticPropertyValue('typed1', 1));
+var_dump($rc->getStaticPropertyValue('typed2'));
+
+?>
+--EXPECT--
+NULL
+Typed property Bug::$typed1 must not be accessed before initialization
+int(1)
+int(3)