diff --git a/src/wp-includes/class-wp-duotone.php b/src/wp-includes/class-wp-duotone.php
index 482ddc6c133ba..08beac3cd4923 100644
--- a/src/wp-includes/class-wp-duotone.php
+++ b/src/wp-includes/class-wp-duotone.php
@@ -1116,9 +1116,12 @@ public static function render_duotone_support( $block_content, $block, $wp_block
 			 */
 
 			$duotone_attr = $block['attrs']['style']['color']['duotone'];
-			$is_preset    = is_string( $duotone_attr ) && self::is_preset( $duotone_attr );
-			$is_css       = is_string( $duotone_attr ) && ! $is_preset;
-			$is_custom    = is_array( $duotone_attr );
+			if ( ! is_string( $duotone_attr ) && ! is_array( $duotone_attr ) ) {
+				return $block_content;
+			}
+
+			$is_preset = is_string( $duotone_attr ) && self::is_preset( $duotone_attr );
+			$is_css    = is_string( $duotone_attr ) && ! $is_preset;
 
 			if ( $is_preset ) {
 
@@ -1136,7 +1139,7 @@ public static function render_duotone_support( $block_content, $block, $wp_block
 
 				// Just block CSS.
 				self::enqueue_block_css( $filter_id, $duotone_selector, $filter_value );
-			} elseif ( $is_custom ) {
+			} else {
 				$slug         = wp_unique_id( sanitize_key( implode( '-', $duotone_attr ) . '-' ) ); // e.g. '000000-ffffff-2'.
 				$filter_id    = self::get_filter_id( $slug ); // e.g. 'wp-duotone-filter-000000-ffffff-2'.
 				$filter_value = self::get_filter_url( $filter_id ); // e.g. 'url(#wp-duotone-filter-000000-ffffff-2)'.
@@ -1148,7 +1151,7 @@ public static function render_duotone_support( $block_content, $block, $wp_block
 				// SVG filter and block CSS.
 				self::enqueue_custom_filter( $filter_id, $duotone_selector, $filter_value, $filter_data );
 			}
-		} elseif ( $has_global_styles_duotone ) {
+		} else {
 			$slug         = $global_styles_block_names[ $block['blockName'] ]; // e.g. 'blue-orange'.
 			$filter_id    = self::get_filter_id( $slug ); // e.g. 'wp-duotone-filter-blue-orange'.
 			$filter_value = self::get_css_var( $slug ); // e.g. 'var(--wp--preset--duotone--blue-orange)'.
diff --git a/tests/phpstan/baselines/variable.undefined.neon b/tests/phpstan/baselines/variable.undefined.neon
index f82fe31a16504..8a2f2a9d8df31 100644
--- a/tests/phpstan/baselines/variable.undefined.neon
+++ b/tests/phpstan/baselines/variable.undefined.neon
@@ -703,11 +703,6 @@ parameters:
 			identifier: variable.undefined
 			count: 5
 			path: ../../../src/wp-includes/class-wp-block-processor.php
-		-
-			message: '#^Variable \$filter_id might not be defined\.$#'
-			identifier: variable.undefined
-			count: 1
-			path: ../../../src/wp-includes/class-wp-duotone.php
 		-
 			message: '#^Variable \$processed_response might not be defined\.$#'
 			identifier: variable.undefined
diff --git a/tests/phpunit/tests/block-supports/duotone.php b/tests/phpunit/tests/block-supports/duotone.php
index f4eaa63faec99..7877254d554da 100644
--- a/tests/phpunit/tests/block-supports/duotone.php
+++ b/tests/phpunit/tests/block-supports/duotone.php
@@ -9,6 +9,38 @@
  */
 
 class Tests_Block_Supports_Duotone extends WP_UnitTestCase {
+	/**
+	 * Tests that unsupported duotone attribute types leave the block unchanged.
+	 *
+	 * @ticket 65817
+	 * @dataProvider data_render_duotone_support_invalid_attribute
+	 * @covers ::render_duotone_support
+	 *
+	 * @param mixed $duotone Invalid duotone attribute value.
+	 */
+	public function test_render_duotone_support_invalid_attribute( $duotone ) {
+		$block         = array(
+			'blockName' => 'core/image',
+			'attrs'     => array( 'style' => array( 'color' => array( 'duotone' => $duotone ) ) ),
+		);
+		$wp_block      = new WP_Block( $block );
+		$block_content = '<figure class="wp-block-image size-full"><img src="/my-image.jpg" /></figure>';
+
+		$this->assertSame( $block_content, WP_Duotone::render_duotone_support( $block_content, $block, $wp_block ) );
+	}
+
+	/**
+	 * @return array
+	 */
+	public function data_render_duotone_support_invalid_attribute() {
+		return array(
+			'false'   => array( false ),
+			'true'    => array( true ),
+			'integer' => array( 1 ),
+			'float'   => array( 1.5 ),
+		);
+	}
+
 	/**
 	 * Tests whether the duotone preset class is added to the block.
 	 *