diff --git a/lib/class-wp-duotone-gutenberg.php b/lib/class-wp-duotone-gutenberg.php
index b18121086bae0..816f7e414ad79 100644
--- a/lib/class-wp-duotone-gutenberg.php
+++ b/lib/class-wp-duotone-gutenberg.php
@@ -530,10 +530,19 @@ private static function get_filter_svg( $filter_id, $colors ) {
 		foreach ( $colors as $color_str ) {
 			$color = self::colord_parse( $color_str );
 
-			$duotone_values['r'][] = $color['r'] / 255;
-			$duotone_values['g'][] = $color['g'] / 255;
-			$duotone_values['b'][] = $color['b'] / 255;
-			$duotone_values['a'][] = $color['a'];
+			if ( null === $color ) {
+				$error_message = sprintf(
+					/* translators: %s: duotone colors */
+					__( '"%s" in theme.json settings.color.duotone is not a hex or rgb string.', 'gutenberg' ),
+					$color_str
+				);
+				_doing_it_wrong( __METHOD__, $error_message, '6.3.0' );
+			} else {
+				$duotone_values['r'][] = $color['r'] / 255;
+				$duotone_values['g'][] = $color['g'] / 255;
+				$duotone_values['b'][] = $color['b'] / 255;
+				$duotone_values['a'][] = $color['a'];
+			}
 		}
 
 		ob_start();