diff --git a/src/sanitizer.js b/src/sanitizer.js
index a72ce13d0758..fd9bcbbd7612 100644
--- a/src/sanitizer.js
+++ b/src/sanitizer.js
@@ -278,6 +278,8 @@ export function sanitizeHtml(html) {
         const attrName = attribs[i];
         const attrValue = attribs[i + 1];
         if (!isValidAttr(tagName, attrName, attrValue)) {
+          user().error(TAG, `Removing "${attrName}" attribute with invalid `
+              + `value in <${tagName} ${attrName}="${attrValue}">.`);
           continue;
         }
         emit(' ');
@@ -357,7 +359,6 @@ export function sanitizeFormattingHtml(html) {
  * @return {boolean}
  */
 export function isValidAttr(tagName, attrName, attrValue) {
-
   // "on*" attributes are not allowed.
   if (startsWith(attrName, 'on') && attrName != 'on') {
     return false;