diff --git a/src/Form/Util/JsonConverter.php b/src/Form/Util/JsonConverter.php
index 1206a9a..409b5da 100644
--- a/src/Form/Util/JsonConverter.php
+++ b/src/Form/Util/JsonConverter.php
@@ -46,7 +46,7 @@ public static function toStdClass(array $data): \stdClass {
       return new \stdClass();
     }
 
-    $result = \json_decode(\json_encode($data, JSON_THROW_ON_ERROR));
+    $result = \json_decode(\json_encode($data, JSON_THROW_ON_ERROR), FALSE, 512, JSON_THROW_ON_ERROR);
     Assertion::isInstanceOf($result, \stdClass::class);
 
     return $result;
diff --git a/src/JsonForms/Definition/Control/ControlDefinition.php b/src/JsonForms/Definition/Control/ControlDefinition.php
index c730641..44f9e8f 100644
--- a/src/JsonForms/Definition/Control/ControlDefinition.php
+++ b/src/JsonForms/Definition/Control/ControlDefinition.php
@@ -159,6 +159,13 @@ public function getEnum(): ?array {
     return $this->propertySchema->enum ?? NULL;
   }
 
+  /**
+   * {@inheritDoc}
+   */
+  public function getKeywordValue(string $keyword, $default = NULL) {
+    return $this->uiSchema->{$keyword} ?? $default;
+  }
+
   public function getLabel(): string {
     return $this->controlSchema->label ?? $this->propertySchema->title ?? ucfirst($this->getPropertyName());
   }
@@ -319,6 +326,9 @@ public function isRequired(): bool {
     return in_array($this->getPropertyName(), $this->objectSchema->required ?? [], TRUE);
   }
 
+  /**
+   * {@inheritDoc}
+   */
   public function withScopePrefix(string $scopePrefix): DefinitionInterface {
     if (NULL !== $this->getScopePrefix()) {
       $scopePrefix = $this->getScopePrefix() . ltrim($scopePrefix, '#');
diff --git a/src/JsonForms/Definition/Custom/CustomDefinition.php b/src/JsonForms/Definition/Custom/CustomDefinition.php
new file mode 100644
index 0000000..988a37d
--- /dev/null
+++ b/src/JsonForms/Definition/Custom/CustomDefinition.php
@@ -0,0 +1,78 @@
+<?php
+
+/*
+ * Copyright (C) 2024 SYSTOPIA GmbH
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published by
+ *  the Free Software Foundation in version 3.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+namespace Drupal\json_forms\JsonForms\Definition\Custom;
+
+use Drupal\json_forms\JsonForms\Definition\DefinitionInterface;
+
+/**
+ * Used for unknown UI schema types.
+ *
+ * @codeCoverageIgnore
+ */
+final class CustomDefinition implements DefinitionInterface {
+
+  private \stdClass $uiSchema;
+
+  private \stdClass $jsonSchema;
+
+  private bool $parentReadonly;
+
+  public function __construct(\stdClass $uiSchema, \stdClass $jsonSchema, bool $parentReadonly) {
+    $this->uiSchema = $uiSchema;
+    $this->jsonSchema = $jsonSchema;
+    $this->parentReadonly = $parentReadonly;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public function getKeywordValue(string $keyword, $default = NULL) {
+    return $this->uiSchema->{$keyword} ?? $default;
+  }
+
+  public function getJsonSchema(): \stdClass {
+    return $this->jsonSchema;
+  }
+
+  public function getUiSchema(): \stdClass {
+    return $this->uiSchema;
+  }
+
+  public function getRule(): ?\stdClass {
+    return $this->uiSchema->rule ?? NULL;
+  }
+
+  public function getType(): string {
+    return $this->uiSchema->type;
+  }
+
+  public function isParentReadonly(): bool {
+    return $this->parentReadonly;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public function withScopePrefix(string $scopePrefix): DefinitionInterface {
+    return new static($this->uiSchema, $this->jsonSchema, $this->parentReadonly);
+  }
+
+}
diff --git a/src/JsonForms/Definition/Markup/MarkupDefinition.php b/src/JsonForms/Definition/Custom/MarkupDefinition.php
similarity index 96%
rename from src/JsonForms/Definition/Markup/MarkupDefinition.php
rename to src/JsonForms/Definition/Custom/MarkupDefinition.php
index e5be14c..dfabca5 100644
--- a/src/JsonForms/Definition/Markup/MarkupDefinition.php
+++ b/src/JsonForms/Definition/Custom/MarkupDefinition.php
@@ -50,10 +50,7 @@ public function getContentMediaType(): string {
   }
 
   /**
-   * @param string $keyword
-   * @param mixed $default
-   *
-   * @return mixed
+   * {@inheritDoc}
    */
   public function getKeywordValue(string $keyword, $default = NULL) {
     return $this->markupSchema->{$keyword} ?? $default;
@@ -75,6 +72,9 @@ public function getType(): string {
     return $this->markupSchema->type;
   }
 
+  /**
+   * {@inheritDoc}
+   */
   public function withScopePrefix(string $scopePrefix): DefinitionInterface {
     return new static($this->markupSchema);
   }
diff --git a/src/JsonForms/Definition/DefinitionFactory.php b/src/JsonForms/Definition/DefinitionFactory.php
index 345646b..7c66002 100644
--- a/src/JsonForms/Definition/DefinitionFactory.php
+++ b/src/JsonForms/Definition/DefinitionFactory.php
@@ -22,6 +22,7 @@
 namespace Drupal\json_forms\JsonForms\Definition;
 
 use Drupal\json_forms\JsonForms\Definition\Control\ControlDefinition;
+use Drupal\json_forms\JsonForms\Definition\Custom\CustomDefinition;
 use Drupal\json_forms\JsonForms\Definition\Layout\LayoutDefinition;
 use Drupal\json_forms\JsonForms\Definition\Markup\MarkupDefinition;
 
@@ -38,7 +39,8 @@ public static function createDefinition(
     if ('Control' === $uiSchema->type) {
       return ControlDefinition::fromJsonSchema($uiSchema, $jsonSchema, $parentUiReadonly);
     }
-    elseif ('Markup' === $uiSchema->type) {
+
+    if ('Markup' === $uiSchema->type) {
       return new MarkupDefinition($uiSchema);
     }
 
@@ -46,7 +48,7 @@ public static function createDefinition(
       return new LayoutDefinition($uiSchema, $jsonSchema, $parentUiReadonly);
     }
 
-    throw new \InvalidArgumentException(sprintf('Invalid type "%s"', $uiSchema->type));
+    return new CustomDefinition($uiSchema, $jsonSchema, $parentUiReadonly);
   }
 
 }
diff --git a/src/JsonForms/Definition/DefinitionInterface.php b/src/JsonForms/Definition/DefinitionInterface.php
index 3db7688..e898295 100644
--- a/src/JsonForms/Definition/DefinitionInterface.php
+++ b/src/JsonForms/Definition/DefinitionInterface.php
@@ -23,12 +23,22 @@
 
 interface DefinitionInterface {
 
+  /**
+   * @param mixed $default
+   *
+   * @return mixed
+   *   The value for the given keyword in the UI schema or $default if not set.
+   */
+  public function getKeywordValue(string $keyword, $default = NULL);
+
+  public function getRule(): ?\stdClass;
+
   public function getType(): string;
 
   /**
-   * @param string $scopePrefix
-   *
    * @return static
+   *   A new definition where the given scope prefix is prepended to the scopes
+   *   of all Controls.
    */
   public function withScopePrefix(string $scopePrefix): self;
 
diff --git a/src/JsonForms/Definition/Layout/LayoutDefinition.php b/src/JsonForms/Definition/Layout/LayoutDefinition.php
index 4c0549e..61d19e5 100644
--- a/src/JsonForms/Definition/Layout/LayoutDefinition.php
+++ b/src/JsonForms/Definition/Layout/LayoutDefinition.php
@@ -58,10 +58,7 @@ public function getLayoutSchema(): \stdClass {
   }
 
   /**
-   * @param string $keyword
-   * @param mixed $default
-   *
-   * @return mixed
+   * {@inheritDoc}
    */
   public function getKeywordValue(string $keyword, $default = NULL) {
     return $this->layoutSchema->{$keyword} ?? $default;
@@ -93,6 +90,9 @@ public function getOptionsValue(string $key, $default = NULL) {
     return $this->layoutSchema->options->{$key} ?? $default;
   }
 
+  /**
+   * {@inheritDoc}
+   */
   public function withScopePrefix(string $scopePrefix): DefinitionInterface {
     $definition = (new \ReflectionClass($this))->newInstanceWithoutConstructor();
     $definition->layoutSchema = $this->layoutSchema;