go-delve/delve · error

malformed map type

Error message

malformed map type

What it means

During classic map iteration, nextBucket loads the tophash, keys, values and overflow fields of a bucket and sanity-checks them. If any is nil the variable is marked Unreadable with a generic 'malformed map type' error, and if any of tophash/keys/values is not a reflect.Array the more specific errMapBucketContentsNotArray is used. It means Delve could not reconstruct a valid bucket from the process memory/DWARF type.

Source

Thrown at pkg/proc/mapiter.go:218

			it.v.Unreadable = field.Unreadable
			return false
		}

		switch f.Name {
		case "tophash": // +rtype -fieldof bmap [8]uint8
			it.tophashes = field
		case "keys":
			it.keys = field
		case "values":
			it.values = field
		case "overflow":
			it.overflow = field.maybeDereference()
		}
	}

	// sanity checks
	if it.tophashes == nil || it.keys == nil || it.values == nil || it.overflow == nil {
		it.v.Unreadable = errors.New("malformed map type")
		return false
	}

	if it.tophashes.Kind != reflect.Array || it.keys.Kind != reflect.Array || it.values.Kind != reflect.Array {
		it.v.Unreadable = errMapBucketContentsNotArray
		return false
	}

	if it.tophashes.Len != it.keys.Len {
		it.v.Unreadable = errMapBucketContentsInconsistentLen
		return false
	}

	if it.values.fieldType.Size() > 0 && it.tophashes.Len != it.values.Len {
		// if the type of the value is zero-sized (i.e. struct{}) then the values
		// array's length is zero.
		it.v.Unreadable = errMapBucketContentsInconsistentLen
		return false

View on GitHub (pinned to a23773e6c3)

Solutions

1. Upgrade Delve to match the Go version of the target binary
2. Re-check target memory readability (for core dumps verify the file is complete and matches the binary)
3. Rebuild the binary with full debug info and retry the map evaluation

// Evaluate the map defensively and surface the Unreadable flag instead of panicking:
v, err := evalMap(expr)
if err != nil { return err }
if v.Unreadable != nil {
    log.Printf("map unreadable: %v (Go/Delve version mismatch likely)", v.Unreadable)
    return fallbackPrintHeaderOnly(v)
}

When it happens

Trigger: Calling next() on a mapIterator (i.e. printing or ranging a map) when a bucket's tophash, keys, values or overflow field loads as nil from target memory — e.g. unreadable memory, an all-zero bucket pointer, or a type layout mismatch.

Common situations: Printing a map in a core dump or a process with partially unreadable memory; Go runtime version whose bucket struct differs from Delve's expectation; corrupted or truncated DWARF for the runtime map types.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• wrong real type for map
• malformed map type: keys, values or tophash of a bucket is n
• malformed map type: inconsistent array length in bucket
• malformed map type: buckets, oldbuckets or overflow field no
• could not load one of the tables

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/f7ae4d1fdedf9780. Report an issue: GitHub.