What causes "map index out of bounds" in go-delve/delve?

Rebuild the debugged binary with the Go version matching your Delve build (map internals are version-specific)

map index out of bounds

Error message

map index out of bounds

What it means

Delve throws this when iterating a Go map's internal buckets while loading its entries: a bucket-chain 'next()' step failed before the requested index was reached. It means the map's internal state (hash buckets/overflow chains) could not be walked far enough to satisfy the requested entry, usually because the runtime map layout assumed by Delve does not match the debugged binary.

Source

Thrown at pkg/proc/variables.go:2026

		v.Unreadable = err
		return 0
	}
	return val
}

func (v *Variable) loadMap(recurseLevel int, cfg LoadConfig) {
	it := v.mapIterator(uint64(cfg.MaxMapBuckets))
	if it == nil {
		return
	}

	if v.Len == 0 || int64(v.mapSkip) >= v.Len || cfg.MaxArrayValues == 0 {
		return
	}

	for skip := 0; skip < v.mapSkip; skip++ {
		if ok := it.next(); !ok {
			v.Unreadable = errors.New("map index out of bounds")
			return
		}
	}

	count := 0
	errcount := 0
	for it.next() {
		key := it.key()
		val := it.value()
		key.loadValueInternal(recurseLevel+1, cfg)
		val.loadValueInternal(recurseLevel+1, cfg)
		if key.Unreadable != nil || val.Unreadable != nil {
			errcount++
		}
		v.Children = append(v.Children, *key, *val)
		count++
		if errcount > maxErrCount {
			break

View on GitHub (pinned to a23773e6c3)

Solutions

Rebuild the debugged binary with the Go version matching your Delve build (map internals are version-specific)
Clear/avoid custom map-skip configuration (config max-array-values / map iteration settings) and re-evaluate the map
Re-evaluate the variable at a fresh stop point so Len and bucket data are re-read consistently
Upgrade Delve to a version supporting your Go runtime's map representation

Example fix

// before
dlv config map-skip 5 // skipping 5 entries on a 3-entry map
// after
dlv config map-skip 0

Defensive patterns

Strategy: fallback

Validate before calling

// Ensure skip is within the reported length before loading
if int64(cfg.mapSkip) >= mapVar.Len {
    return fmt.Errorf("mapSkip %d exceeds map length %d", cfg.mapSkip, mapVar.Len)
}

Type guard

func mapRangeIsValid(v *proc.Variable, skip int) bool {
    return v.Kind == reflect.Map && int64(skip) < v.Len
}

Try / catch

v, err := loadMap(m)
if err != nil && strings.Contains(err.Error(), "map index out of bounds") {
    // retry once with a fresh stop state / skip=0
cfg.mapSkip = 0
    v, err = loadMap(m)
}

Prevention

Match Delve version to the target Go version (map internals change, e.g. swiss maps)
Avoid mutating maps concurrently while stopped at breakpoints you inspect
Don't set map-skip beyond the map's current length
Re-evaluate the variable after each stop instead of caching Len

When it happens

Trigger: Calling LoadValue/LoadMapEntry on a map variable where v.mapSkip (configured map start position) is >= the number of entries actually iterable, or where the map's bucket layout read from memory is inconsistent with v.Len.

Common situations: Debugging Go binaries built with a different Go version than the Delve-supported swiss-map/bucket layout; inspecting maps mid-mutation in a stopped process; using the 'map-skip' config on a map whose length changed after Len was captured.

Related errors

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/e375c4104b81d92c. Report an issue: GitHub.