--- a/devtools/client/webconsole/reducers/messages.js
+++ b/devtools/client/webconsole/reducers/messages.js
@@ -842,17 +842,25 @@ function isTextInStackTrace(text, stackt
 /**
 * Returns true if given text is included in `messageText` field.
 */
 function isTextInMessageText(text, messageText) {
   if (!messageText) {
     return false;
   }
 
-  return messageText.toLocaleLowerCase().includes(text.toLocaleLowerCase());
+  if (typeof messageText === "string") {
+    return messageText.toLocaleLowerCase().includes(text.toLocaleLowerCase());
+  }
+
+  if (messageText.type === "longString") {
+    return messageText.initial.toLocaleLowerCase().includes(text.toLocaleLowerCase());
+  }
+
+  return true;
 }
 
 /**
 * Returns true if given text is included in notes.
 */
 function isTextInNotes(text, notes) {
   if (!Array.isArray(notes)) {
     return false;