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this.req.headers.upgrade is undefined when trying to call toLowerCase()

Freshover 14 years ago
Mar 14, 20260 views
Confidence Score85%
85%

Problem

Hi guys, First of all apologies I can't be more helpful with this bug report - it occurred on a production server thus logging was down to its minimum level. I'm also not very familiar with all the magic which socket.io provides, but here it is anyway. All of a sudden, my server app crashed, and the following was all I had in the log: [code block] It looks like something unexpected happened when a new client tried to connect to the server - with my poor HTTP knowledge it looks like socket.io is always expecting an upgrade header which it didn't receive? I suspect the fact that I am _only_ supporting the websocket transport (e.g. io.set('transports', ['websocket']) may be relevant - perhaps an old client tried to connect which refused to send the upgrade header. Not sure. Using latest release, 0.8.6 for both client and server: [code block] Anything else I can provide you with to help, just shout.

Error Output

Error: Cannot call method 'toLowerCase' of undefined

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1 Fix

Canonical Fix
Moderate Confidence Fix
84% confidence100% success rate1 verificationLast verified Mar 14, 2026

Solution: this.req.headers.upgrade is undefined when trying to call toLowerCase()

Low Risk

In case it's of any relevance, I'm connecting on the client side simply using: [code block] Haven't been able to reproduce this behaviour yet in any browser.

84

Trust Score

1 verification

100% success
  1. 1

    In case it's of any relevance, I'm connecting on the client side simply using:

    In case it's of any relevance, I'm connecting on the client side simply using:

    text
    var socket = io.connect();
  2. 2

    Haven't been able to reproduce this behaviour yet in any browser.

    Haven't been able to reproduce this behaviour yet in any browser.

Validation

Resolved in socketio/socket.io GitHub issue #602. Community reactions: 0 upvotes.

Verification Summary

Worked: 1
Last verified Mar 14, 2026

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Environment

Submitted by

AC

Alex Chen

2450 rep

Tags

socket.iowebsocketrealtimebug