diff --git a/static/js/compose.js b/static/js/compose.js
index 019f915747..ad8e5e4851 100644
--- a/static/js/compose.js
+++ b/static/js/compose.js
@@ -770,14 +770,14 @@ function validate_private_message() {
 
     let context = {};
     if (invalid_recipients.length === 1) {
-        context = {recipient: invalid_recipients.join()};
+        context = {recipient: invalid_recipients.join(",")};
         compose_error(
             $t_html({defaultMessage: "The recipient {recipient} is not valid"}, context),
             $("#private_message_recipient"),
         );
         return false;
     } else if (invalid_recipients.length > 1) {
-        context = {recipients: invalid_recipients.join()};
+        context = {recipients: invalid_recipients.join(",")};
         compose_error(
             $t_html({defaultMessage: "The recipients {recipients} are not valid"}, context),
             $("#private_message_recipient"),
diff --git a/static/js/filter.js b/static/js/filter.js
index 4fbdf8b265..c786d81812 100644
--- a/static/js/filter.js
+++ b/static/js/filter.js
@@ -550,9 +550,9 @@ export class Filter {
                 case "streams-public":
                     return "/#narrow/streams/public";
                 case "pm-with":
-                    // join is used to transform the array to a comma separated string
                     return (
-                        "/#narrow/pm-with/" + people.emails_to_slug(this.operands("pm-with").join())
+                        "/#narrow/pm-with/" +
+                        people.emails_to_slug(this.operands("pm-with").join(","))
                     );
                 // TODO: It is ambiguous how we want to handle the 'sender' case,
                 // we may remove it in the future based on design decisions