diff --git a/src/Generator/AbstractGenerator.php b/src/Generator/AbstractGenerator.php
index 3f50dc3..a6c9d16 100644
--- a/src/Generator/AbstractGenerator.php
+++ b/src/Generator/AbstractGenerator.php
@@ -36,7 +36,7 @@ protected function dumpPHPType(Type $type, callable $importer) : string
         if ($type instanceof Type\NullableType) {
             $wrappedType = $type->getWrappedType();
 
-            if ($wrappedType instanceof Type\UnionType) {
+            if ($wrappedType instanceof Type\UnionType || $wrappedType instanceof ScalarType) {
                 return sprintf('null|%s', $this->dumpPHPType($wrappedType, $importer));
             }
 
diff --git a/tests/Generator/AbstractGeneratorTest.php b/tests/Generator/AbstractGeneratorTest.php
new file mode 100644
index 0000000..6d7709f
--- /dev/null
+++ b/tests/Generator/AbstractGeneratorTest.php
@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\Generator;
+
+use PHPUnit\Framework\TestCase;
+use Ruudk\GraphQLCodeGenerator\Config\Config;
+use Ruudk\GraphQLCodeGenerator\Type\ScalarType;
+use Symfony\Component\TypeInfo\Type as SymfonyType;
+
+final class AbstractGeneratorTest extends TestCase
+{
+    public function testNullableCustomScalarUsesExplicitNullUnionInsteadOfNullableShorthand() : void
+    {
+        $generator = new class(Config::create(schema: '', projectDir: '', queriesDir: '', outputDir: '', namespace: 'Test\\Generated', client: 'Test\\Client')) extends AbstractGenerator {
+            public function dump(SymfonyType $type) : string
+            {
+                return $this->dumpPHPType($type, static fn(string $className) => $className);
+            }
+        };
+
+        self::assertSame(
+            'null|int|string|float|bool',
+            $generator->dump(SymfonyType::nullable(new ScalarType())),
+            'Nullable custom scalar output should not use ? with a multi-type union.',
+        );
+    }
+}