panic: sync: negative WaitGroup counter Go Origin: Application & API Last updated 4 Nov 2026

Go: panic: sync: negative WaitGroup counter

wg.Done() was called more times than wg.Add().

Exact message you may see panic: sync: negative WaitGroup countersync: negative WaitGroup countersync: WaitGroup is reused before previous Wait has returnedWaitGroup misuse: Add called concurrently with Wait
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Seen on: Go

Meaning

Each goroutine should call Done exactly once, usually via defer wg.Done(), after a matching Add(1) made before the goroutine starts. Calling Add inside the goroutine races with Wait. The related panic "WaitGroup is reused before previous Wait has returned" signals reuse bugs.

Common causes

  • Done() called twice per goroutine
  • Add(n) smaller than the number of goroutines
  • Add called inside the goroutine
  • WaitGroup copied by value (passed without pointer)
  • Reuse across loops

⚡ Quick fix

  1. wg.Add(1) before go func(){ defer wg.Done() … }()
  2. Pass *sync.WaitGroup, not a copy
  3. Use go vet (copylocks) to catch copies

Detailed fix by platform

Go

  1. go vet ./... reports passing sync.WaitGroup by value

How to diagnose

  1. Pairing — Add and Done balanced?
  2. Copy — Passed by pointer?
  3. Timing — Add before go?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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