diff --git a/src/wp-includes/theme-previews.php b/src/wp-includes/theme-previews.php
index d9325ddbf36de..e8e4a6a8c8d5d 100644
--- a/src/wp-includes/theme-previews.php
+++ b/src/wp-includes/theme-previews.php
@@ -70,7 +70,7 @@ function wp_block_theme_activate_nonce() {
 	$nonce_handle = 'switch-theme_' . wp_get_theme_preview_path();
 	?>
 	<script type="text/javascript">
-		window.WP_BLOCK_THEME_ACTIVATE_NONCE = '<?php echo wp_create_nonce( $nonce_handle ); ?>';
+		window.WP_BLOCK_THEME_ACTIVATE_NONCE = <?php echo wp_json_encode( wp_create_nonce( $nonce_handle ) ); ?>;
 	</script>
 	<?php
 }