diff --git a/src/static/js/chat.js b/src/static/js/chat.js
index 8cc1d76c1..0b31428b6 100755
--- a/src/static/js/chat.js
+++ b/src/static/js/chat.js
@@ -132,6 +132,16 @@ var chat = (function()
       var timeStr = hours + ":" + minutes;
 
       //create the authorclass
+      if (!msg.userId) {
+        /*
+         * If, for a bug or a database corruption, the message coming from the
+         * server does not contain the userId field (see for example #3731),
+         * let's be defensive and replace it with "unknown".
+         */
+        msg.userId = "unknown";
+        console.warn('The "userId" field of a chat message coming from the server was not present. Replacing with "unknown". This may be a bug or a database corruption.');
+      }
+
       var authorClass = "author-" + msg.userId.replace(/[^a-y0-9]/g, function(c)
       {
         if (c == ".") return "-";