diff --git a/lib/class-wp-theme-json-gutenberg.php b/lib/class-wp-theme-json-gutenberg.php
index f3ed36dce18e57..277a967c208242 100644
--- a/lib/class-wp-theme-json-gutenberg.php
+++ b/lib/class-wp-theme-json-gutenberg.php
@@ -971,7 +971,7 @@ public function get_settings() {
 	 *
 	 * @return string The processed CSS.
 	 */
-	public function process_blocks_custom_css( $css, $selector ) {
+	protected function process_blocks_custom_css( $css, $selector ) {
 		$processed_css = '';
 
 		// Split CSS nested rules.
diff --git a/phpunit/class-wp-theme-json-test.php b/phpunit/class-wp-theme-json-test.php
index 4aca6ad5c3bf1f..423b8a6ff41546 100644
--- a/phpunit/class-wp-theme-json-test.php
+++ b/phpunit/class-wp-theme-json-test.php
@@ -2010,8 +2010,10 @@ public function test_process_blocks_custom_css( $input, $expected ) {
 				'styles'  => array(),
 			)
 		);
+		$reflection = new ReflectionMethod( $theme_json, 'process_blocks_custom_css' );
+		$reflection->setAccessible( true );
 
-		$this->assertEquals( $expected, $theme_json->process_blocks_custom_css( $input['css'], $input['selector'] ) );
+		$this->assertEquals( $expected, $reflection->invoke( $theme_json, $input['css'], $input['selector'] ) );
 	}
 
 	/**