diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0ba97445..1d7d467a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ Latest
 * [#189](https://github.com/cleverage/process-bundle/issues/189) Fix EventDispatcherTask: dispatch the event under the `event_name` option (regression since v4.0). `event_name` is now optional: when `null` (default), the event is dispatched under its class name. Update documentation, add tests.
 * [#198](https://github.com/cleverage/process-bundle/issues/198) Fix DateParserTransformer: a `\DateTimeImmutable` input is converted to a `\DateTime` instead of throwing a `TypeError`. Update documentation, add tests.
 * [#199](https://github.com/cleverage/process-bundle/issues/199) Fix PregFilterTransformer: an array `replacement` is no longer cast to the string `"Array"`, and requires an array `pattern`. Update documentation, add tests.
+* [#200](https://github.com/cleverage/process-bundle/issues/200) Fix GenericTransformer: a contextual option declared with `required: false` and no default can now be used; its placeholders are replaced by `null` when omitted. Update documentation, add tests.
 
 ## Deprecated
 * [#189](https://github.com/cleverage/process-bundle/issues/189) EventDispatcherTask: when `event_name` is set, listening to `CleverAge\ProcessBundle\Event\EventDispatcherTaskEvent` is deprecated (the event is still dispatched under its class name, with an `E_USER_DEPRECATED` error, if it has listeners). Listen to the configured `event_name` instead: the BC layer will be removed in v6.0.
diff --git a/docs/reference/03-generic_transformers_definition.md b/docs/reference/03-generic_transformers_definition.md
index 1883a819..013a8f0d 100644
--- a/docs/reference/03-generic_transformers_definition.md
+++ b/docs/reference/03-generic_transformers_definition.md
@@ -33,7 +33,8 @@ For each contextual option, you can define
 | `default_is_null` | `bool` | | `false` | If you need `null` to be the default value, use this option |
 
 Note that an option with a default value is still required by default, which has no effect since the default is used:
-set `required: false` for an option without default value that may be omitted.
+set `required: false` for an option without default value that may be omitted: when omitted, its placeholders are
+replaced by `null`.
 
 The `transformers` list uses the same syntax as any other transformer using a sub-list of transformers (see
 [TransformerTrait](traits/transformer_trait.md)).
diff --git a/docs/reference/transformers/generic_transformer.md b/docs/reference/transformers/generic_transformer.md
index 135967d7..a8b9ec0a 100644
--- a/docs/reference/transformers/generic_transformer.md
+++ b/docs/reference/transformers/generic_transformer.md
@@ -102,5 +102,5 @@ Notes
 -----
 
 * Contextual options are transformer options, unrelated to the process context (`-c key:value`).
-* An option declared with `required: false` and no default is neither required nor defined, so it cannot be used:
-  always give such an option a `default` or `default_is_null: true`.
+* An option declared with `required: false` and no default may be omitted: its placeholders are then replaced by
+  `null`.
diff --git a/src/Transformer/GenericTransformer.php b/src/Transformer/GenericTransformer.php
index f1863aa4..bf53d955 100644
--- a/src/Transformer/GenericTransformer.php
+++ b/src/Transformer/GenericTransformer.php
@@ -79,6 +79,7 @@ public function configureInitialOptions(OptionsResolver $resolver): void
     public function configureOptions(OptionsResolver $resolver): void
     {
         foreach ($this->contextualOptions as $option => $optionConfig) {
+            $resolver->setDefined($option);
             if (null !== $optionConfig['default'] || $optionConfig['default_is_null']) {
                 $resolver->setDefault($option, $optionConfig['default']);
             }
@@ -118,7 +119,10 @@ public function normalizeTransformerOptions(Options $options, array $transformer
     {
         $contextualizedOptionValues = [];
         foreach ($this->contextualOptions as $contextualOption => $contextualOptionConfig) {
-            $contextualizedOptionValues[$contextualOption] = $options[$contextualOption];
+            // An optional contextual option without default value is replaced by null when not provided
+            $contextualizedOptionValues[$contextualOption] = $options->offsetExists($contextualOption)
+                ? $options[$contextualOption]
+                : null;
         }
 
         return $this->contextualOptionResolver->contextualizeOptions($transformerOptions, $contextualizedOptionValues);
diff --git a/tests/Transformer/GenericTransformerTest.php b/tests/Transformer/GenericTransformerTest.php
new file mode 100644
index 00000000..001a9183
--- /dev/null
+++ b/tests/Transformer/GenericTransformerTest.php
@@ -0,0 +1,110 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the CleverAge/ProcessBundle package.
+ *
+ * Copyright (c) Clever-Age
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CleverAge\ProcessBundle\Tests\Transformer;
+
+use CleverAge\ProcessBundle\Context\ContextualOptionResolver;
+use CleverAge\ProcessBundle\Registry\TransformerRegistry;
+use CleverAge\ProcessBundle\Transformer\CallbackTransformer;
+use CleverAge\ProcessBundle\Transformer\GenericTransformer;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\OptionsResolver\Exception\MissingOptionsException;
+use Symfony\Component\OptionsResolver\OptionsResolver;
+
+#[\PHPUnit\Framework\Attributes\CoversClass(GenericTransformer::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(ContextualOptionResolver::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(TransformerRegistry::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(CallbackTransformer::class)]
+class GenericTransformerTest extends TestCase
+{
+    public function testRequiredOptionMustBeProvided(): void
+    {
+        $transformer = $this->createSubstrTransformer(['offset' => ['required' => true]]);
+
+        $this->expectException(MissingOptionsException::class);
+
+        $this->resolveOptions($transformer, []);
+    }
+
+    public function testOptionWithDefault(): void
+    {
+        $transformer = $this->createSubstrTransformer([
+            'offset' => ['default' => 2],
+            'length' => ['default_is_null' => true],
+        ]);
+
+        $this->assertSame('llo', $transformer->transform('hello', $this->resolveOptions($transformer, [])));
+        $this->assertSame('ll', $transformer->transform('hello', $this->resolveOptions($transformer, ['length' => 2])));
+    }
+
+    public function testOptionalOptionWithoutDefaultCanBeProvided(): void
+    {
+        $transformer = $this->createSubstrTransformer([
+            'offset' => ['required' => true],
+            'length' => ['required' => false],
+        ]);
+
+        $options = $this->resolveOptions($transformer, ['offset' => 1, 'length' => 3]);
+
+        $this->assertSame('ell', $transformer->transform('hello', $options));
+    }
+
+    public function testOptionalOptionWithoutDefaultIsNullWhenOmitted(): void
+    {
+        $transformer = $this->createSubstrTransformer([
+            'offset' => ['required' => true],
+            'length' => ['required' => false],
+        ]);
+
+        $options = $this->resolveOptions($transformer, ['offset' => 1]);
+
+        $this->assertSame('ello', $transformer->transform('hello', $options));
+    }
+
+    /**
+     * Generic transformer applying substr($value, {{ offset }}, {{ length }}).
+     *
+     * @param array<string, array<string, mixed>> $contextualOptions
+     */
+    private function createSubstrTransformer(array $contextualOptions): GenericTransformer
+    {
+        $registry = new TransformerRegistry();
+        $registry->addTransformer(new CallbackTransformer());
+
+        $transformer = new GenericTransformer(new ContextualOptionResolver(), $registry);
+        $transformer->initialize('substr', [
+            'contextual_options' => $contextualOptions,
+            'transformers' => [
+                'callback' => [
+                    'callback' => 'substr',
+                    'right_parameters' => ['{{ offset }}', '{{ length }}'],
+                ],
+            ],
+        ]);
+
+        return $transformer;
+    }
+
+    /**
+     * @param array<string, mixed> $options
+     *
+     * @return array<string, mixed>
+     */
+    private function resolveOptions(GenericTransformer $transformer, array $options): array
+    {
+        $resolver = new OptionsResolver();
+        $transformer->configureOptions($resolver);
+
+        return $resolver->resolve($options);
+    }
+}