diff --git a/Civi/RemoteTools/JsonSchema/FormSpec/Factory/OptionFieldFactory.php b/Civi/RemoteTools/JsonSchema/FormSpec/Factory/OptionFieldFactory.php
index 766bb6b..2af1648 100644
--- a/Civi/RemoteTools/JsonSchema/FormSpec/Factory/OptionFieldFactory.php
+++ b/Civi/RemoteTools/JsonSchema/FormSpec/Factory/OptionFieldFactory.php
@@ -32,8 +32,16 @@ public function createSchema(AbstractFormField $field): JsonSchema {
     /** @var \Civi\RemoteTools\Form\FormSpec\Field\AbstractOptionField $field */
     $keywords = [
       'type' => ['string', 'integer'],
-      'oneOf' => JsonSchemaUtil::buildTitledOneOf($field->getOptions()),
     ];
+
+    // 'oneOf' must not be empty.
+    if ($field->getOptions() !== []) {
+      $keywords['oneOf'] = JsonSchemaUtil::buildTitledOneOf($field->getOptions());
+    }
+    elseif (!$field->isNullable()) {
+      $keywords['oneOf'] = JsonSchema::fromArray(['const' => NULL]);
+    }
+
     if ($field->isNullable()) {
       $keywords['type'][] = 'null';
       $keywords['oneOf'][] = JsonSchema::fromArray(['const' => NULL]);
diff --git a/Civi/RemoteTools/JsonSchema/Util/JsonSchemaUtil.php b/Civi/RemoteTools/JsonSchema/Util/JsonSchemaUtil.php
index 6122204..4a15b50 100644
--- a/Civi/RemoteTools/JsonSchema/Util/JsonSchemaUtil.php
+++ b/Civi/RemoteTools/JsonSchema/Util/JsonSchemaUtil.php
@@ -24,10 +24,12 @@
 final class JsonSchemaUtil {
 
   /**
-   * @phpstan-param array<int|string, string> $titles
+   * The 'oneOf' keyword must not be empty.
+   *
+   * @phpstan-param non-empty-array<int|string, string> $titles
    *   Allowed values mapped to titles.
    *
-   * @phpstan-return array<JsonSchema> To be used as value of "oneOf" keyword.
+   * @phpstan-return non-empty-array<JsonSchema> To be used as value of "oneOf" keyword.
    */
   public static function buildTitledOneOf(array $titles): array {
     $oneOf = [];