diff --git a/inc/classes/metform/class-metform-integration.php b/inc/classes/metform/class-metform-integration.php
index 31d96c2e7..0a6d5b32d 100644
--- a/inc/classes/metform/class-metform-integration.php
+++ b/inc/classes/metform/class-metform-integration.php
@@ -95,7 +95,7 @@ public function add_additional_css_for_godam_player() {
 	 * @since 1.4.0
 	 *
 	 * @param string $layer Layer name.
-	 * @param string $layer_id Layer ID.
+	 * @param int    $layer_id Layer ID.
 	 *
 	 * @return void
 	 */
@@ -121,13 +121,13 @@ public function add_css_for_the_layer_inside_iframe( $layer, $layer_id ) {
 	 * @since 1.4.0
 	 *
 	 * @param string $layer Layer name.
-	 * @param string $layer_id Layer ID.
+	 * @param int    $layer_id Layer ID.
 	 *
 	 * @return void
 	 */
 	public function render_layer_form_for_video_editor( $layer, $layer_id ) {
 		if ( 'metform' === $layer && ! empty( $layer_id ) ) {
-			echo do_shortcode( "[metform form_id='{$layer_id}']" ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+			echo do_shortcode( sprintf( "[metform form_id='%d']", absint( $layer_id ) ) ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
 		}
 	}
 }
diff --git a/inc/classes/ninja-forms/class-ninja-forms-integration.php b/inc/classes/ninja-forms/class-ninja-forms-integration.php
index 9b219a603..0568d4d1b 100644
--- a/inc/classes/ninja-forms/class-ninja-forms-integration.php
+++ b/inc/classes/ninja-forms/class-ninja-forms-integration.php
@@ -121,7 +121,7 @@ public function add_additional_css_for_godam_player() {
 	 * @since 1.4.0
 	 *
 	 * @param string $layer Layer name.
-	 * @param string $layer_id Layer ID.
+	 * @param int    $layer_id Layer ID.
 	 *
 	 * @return void
 	 */
@@ -150,13 +150,13 @@ public function add_css_for_the_layer_inside_iframe( $layer, $layer_id ) {
 	 * @since 1.4.0
 	 *
 	 * @param string $layer Layer name.
-	 * @param string $layer_id Layer ID.
+	 * @param int    $layer_id Layer ID.
 	 *
 	 * @return void
 	 */
 	public function render_layer_form_for_video_editor( $layer, $layer_id ) {
 		if ( 'ninja-forms' === $layer && ! empty( $layer_id ) ) {
-			echo do_shortcode( "[ninja_form id='{$layer_id}']" ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+			echo do_shortcode( sprintf( "[ninja_form id='%d']", absint( $layer_id ) ) ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
 		}
 	}
 
diff --git a/inc/templates/render-video-editor-form-layer.php b/inc/templates/render-video-editor-form-layer.php
index 0ba929bc7..d8830590e 100644
--- a/inc/templates/render-video-editor-form-layer.php
+++ b/inc/templates/render-video-editor-form-layer.php
@@ -19,8 +19,8 @@
 // We need to remove the admin bar for this template to avoid to display it on the video editor page.
 add_filter( 'show_admin_bar', '__return_false' ); // phpcs:ignore WordPressVIPMinimum.UserExperience.AdminBarRemoval.RemovalDetected
 
-$godam_layer    = get_query_var( 'rtgodam-render-layer' );
-$godam_layer_id = get_query_var( 'rtgodam-layer-id' );
+$godam_layer    = sanitize_key( get_query_var( 'rtgodam-render-layer' ) );
+$godam_layer_id = absint( get_query_var( 'rtgodam-layer-id' ) );
 ?>
 
 <!DOCTYPE html>
@@ -38,7 +38,7 @@
 	 * @since 1.4.0
 	 *
 	 * @param string $godam_layer The type of layer being rendered.
-	 * @param string $godam_layer_id The ID of the layer being rendered.
+	 * @param int    $godam_layer_id The ID of the layer being rendered.
 	 */
 	do_action( 'rtgodam_render_layer_for_video_editor_before', $godam_layer, $godam_layer_id );
 	?>
@@ -55,7 +55,7 @@
 				 * @since 1.4.0
 				 *
 				 * @param string $godam_layer The type of layer being rendered.
-				 * @param string $godam_layer_id The ID of the layer being rendered.
+				 * @param int    $godam_layer_id The ID of the layer being rendered.
 				 */
 				do_action( 'rtgodam_render_layer_for_video_editor', $godam_layer, $godam_layer_id );
 			?>
@@ -67,7 +67,7 @@
 		 * @since 1.4.0
 		 *
 		 * @param string $godam_layer The type of layer being rendered.
-		 * @param string $godam_layer_id The ID of the layer being rendered.
+		 * @param int    $godam_layer_id The ID of the layer being rendered.
 		 */
 		do_action( 'rtgodam_render_layer_for_video_editor_after', $godam_layer, $godam_layer_id );
 	?>
diff --git a/tests/php/FormLayerPreviewIdTest.php b/tests/php/FormLayerPreviewIdTest.php
new file mode 100644
index 000000000..cab76df9d
--- /dev/null
+++ b/tests/php/FormLayerPreviewIdTest.php
@@ -0,0 +1,101 @@
+<?php
+/**
+ * Unit tests for the id handed to the form layer preview.
+ *
+ * The preview page reads `rtgodam-layer-id` from the address. Whatever the
+ * address holds, the Ninja Forms and MetForm integrations must only ever build
+ * their shortcode from a whole number. The template is rendered for real and
+ * its output hooks drive the real integration callbacks; only the WordPress
+ * template tags and do_shortcode() are stubbed.
+ *
+ * @package GoDAM
+ */
+
+namespace RTGODAM\Tests;
+
+use PHPUnit\Framework\TestCase;
+use RTGODAM\Inc\Metform\Metform_Integration;
+use RTGODAM\Inc\Ninja_Forms\Ninja_Forms_Integration;
+
+/**
+ * @covers \RTGODAM\Inc\Metform\Metform_Integration::render_layer_form_for_video_editor
+ * @covers \RTGODAM\Inc\Ninja_Forms\Ninja_Forms_Integration::render_layer_form_for_video_editor
+ */
+class FormLayerPreviewIdTest extends TestCase {
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		require_once dirname( __DIR__ ) . '/stubs/form-layer-preview-functions.php';
+		require_once dirname( __DIR__, 2 ) . '/inc/classes/metform/class-metform-integration.php';
+		require_once dirname( __DIR__, 2 ) . '/inc/classes/ninja-forms/class-ninja-forms-integration.php';
+
+		$GLOBALS['rtgodam_hooks'] = array();
+		$GLOBALS['rtgodam_stub']  = array( 'shortcodes' => array() );
+
+		// Register only the preview callbacks, on constructor-less instances.
+		foreach ( array( Metform_Integration::class, Ninja_Forms_Integration::class ) as $class ) {
+			$object = ( new \ReflectionClass( $class ) )->newInstanceWithoutConstructor();
+			add_action( 'rtgodam_render_layer_for_video_editor_before', array( $object, 'add_css_for_the_layer_inside_iframe' ), 10, 2 );
+			add_action( 'rtgodam_render_layer_for_video_editor', array( $object, 'render_layer_form_for_video_editor' ), 10, 2 );
+		}
+	}
+
+	protected function tearDown(): void {
+		unset( $GLOBALS['rtgodam_stub'], $GLOBALS['rtgodam_hooks'] );
+		parent::tearDown();
+	}
+
+	/**
+	 * Render the preview template for the given query vars and return the
+	 * shortcode strings that reached do_shortcode().
+	 *
+	 * @param string $layer    Value of `rtgodam-render-layer`.
+	 * @param mixed  $layer_id Value of `rtgodam-layer-id`.
+	 * @return string[]
+	 */
+	private function render_preview( $layer, $layer_id ) {
+		$GLOBALS['rtgodam_stub']['query_vars'] = array(
+			'rtgodam-render-layer' => $layer,
+			'rtgodam-layer-id'     => $layer_id,
+		);
+		$GLOBALS['rtgodam_stub']['shortcodes'] = array();
+
+		ob_start();
+		include dirname( __DIR__, 2 ) . '/inc/templates/render-video-editor-form-layer.php';
+		ob_end_clean();
+
+		return $GLOBALS['rtgodam_stub']['shortcodes'];
+	}
+
+	/** A plain numeric id still builds the same shortcodes as before. */
+	public function test_numeric_id_builds_the_expected_shortcodes() {
+		$this->assertSame( array( "[ninja_form id='42']" ), $this->render_preview( 'ninja-forms', '42' ) );
+		$this->assertSame( array( "[metform form_id='42']" ), $this->render_preview( 'metform', '42' ) );
+	}
+
+	/** Anything after the leading digits never reaches the shortcode. */
+	public function test_non_numeric_id_reaches_the_shortcode_as_an_integer() {
+		$value = "7'][gd1_marker]x";
+
+		$this->assertSame( array( "[ninja_form id='7']" ), $this->render_preview( 'ninja-forms', $value ) );
+		$this->assertSame( array( "[metform form_id='7']" ), $this->render_preview( 'metform', $value ) );
+	}
+
+	/** An id with no leading digits becomes 0 and renders nothing. */
+	public function test_id_without_digits_renders_no_shortcode() {
+		$this->assertSame( array(), $this->render_preview( 'ninja-forms', "'][gd1_marker]" ) );
+		$this->assertSame( array(), $this->render_preview( 'metform', "'][gd1_marker]" ) );
+	}
+
+	/** The callbacks also cast on their own, for callers other than the template. */
+	public function test_callbacks_cast_a_string_id_when_called_directly() {
+		$ninja   = ( new \ReflectionClass( Ninja_Forms_Integration::class ) )->newInstanceWithoutConstructor();
+		$metform = ( new \ReflectionClass( Metform_Integration::class ) )->newInstanceWithoutConstructor();
+
+		$ninja->render_layer_form_for_video_editor( 'ninja-forms', "9'][gd1_marker]" );
+		$metform->render_layer_form_for_video_editor( 'metform', "9'][gd1_marker]" );
+
+		$this->assertSame( array( "[ninja_form id='9']", "[metform form_id='9']" ), $GLOBALS['rtgodam_stub']['shortcodes'] );
+	}
+}
diff --git a/tests/stubs/form-layer-preview-functions.php b/tests/stubs/form-layer-preview-functions.php
new file mode 100644
index 000000000..e9739c18b
--- /dev/null
+++ b/tests/stubs/form-layer-preview-functions.php
@@ -0,0 +1,64 @@
+<?php
+/**
+ * Narrow WordPress stubs for FormLayerPreviewIdTest.
+ *
+ * Just enough of the template tags used by inc/templates/render-video-editor-form-layer.php
+ * to render it outside WordPress. Query vars come from `$GLOBALS['rtgodam_stub']['query_vars']`
+ * and every shortcode string handed to do_shortcode() is recorded in
+ * `$GLOBALS['rtgodam_stub']['shortcodes']`.
+ *
+ * @package GoDAM
+ */
+
+if ( ! function_exists( 'get_query_var' ) ) {
+
+	/**
+	 * @param string $var_name Query var name.
+	 * @return mixed
+	 */
+	function get_query_var( $var_name ) { // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- stub mirroring the WP function.
+		return $GLOBALS['rtgodam_stub']['query_vars'][ $var_name ] ?? '';
+	}
+}
+
+if ( ! function_exists( 'sanitize_key' ) ) {
+
+	/**
+	 * @param string $key Input.
+	 * @return string
+	 */
+	function sanitize_key( $key ) { // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- stub mirroring the WP function.
+		return preg_replace( '/[^a-z0-9_\-]/', '', strtolower( (string) $key ) );
+	}
+}
+
+if ( ! function_exists( 'do_shortcode' ) ) {
+
+	/**
+	 * Record the shortcode string instead of running it.
+	 *
+	 * @param string $content Shortcode content.
+	 * @return string
+	 */
+	function do_shortcode( $content ) { // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- stub mirroring the WP function.
+		$GLOBALS['rtgodam_stub']['shortcodes'][] = $content;
+		return '';
+	}
+}
+
+foreach ( array( 'language_attributes', 'wp_head', 'wp_body_open', 'wp_footer', 'body_class' ) as $rtgodam_noop ) {
+	if ( ! function_exists( $rtgodam_noop ) ) {
+		eval( 'function ' . $rtgodam_noop . '() {}' ); // phpcs:ignore Squiz.PHP.Eval.Discouraged -- test-only no-op template tags.
+	}
+}
+
+if ( ! function_exists( 'bloginfo' ) ) {
+
+	/**
+	 * @param string $show Ignored.
+	 * @return void
+	 */
+	function bloginfo( $show = '' ) { // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound, Generic.CodeAnalysis.UnusedFunctionParameter.Found -- stub mirroring the WP function.
+		echo 'UTF-8';
+	}
+}