golang/go · error

invalid NUL in =... value

Error message

invalid NUL in %s=... value

What it means

Returned by checkEnvWrite when the value written via `go env -w` contains a NUL byte (\x00). NUL is illegal in environment values on most platforms and would silently truncate the value or break file parsing.

Solutions

1. Strip NUL bytes from the value before invoking `go env -w`.
2. Validate the input source — values should be human-readable text.
3. Reject binary input at the trust boundary of your config tooling.

Example fix

// before
v := string(binaryBuf) // may contain \x00
goEnvW(key, v)
// after — strip and validate
v = strings.ReplaceAll(v, "\x00", "")
if strings.ContainsRune(v, 0) { return fmt.Errorf("nul") }
goEnvW(key, v)

import "strings"

if strings.ContainsRune(val, '\x00') {
    return fmt.Errorf("value for %s contains NUL", key)
}

When it happens

Trigger: Any `go env -w KEY=...` value containing a literal NUL byte, typically from binary input, embedded C strings, or a corrupt env value.

Common situations: Scripts forwarding bytes from binary blobs; values constructed by string concatenation over fixed-size buffers; malicious/accidental control characters in templated config.

Related errors

• invalid newline in =... value
• GOMODCACHE entry is relative; must be absolute path
• GOPATH entry cannot start with shell metacharacter '~
• GOPATH entry is relative; must be absolute path
• invalid

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/ce18b62707024d04. Report an issue: GitHub.

Appendix: source

Thrown at src/cmd/go/internal/envcmd/env.go:697

			break
		}
		args, err := quoted.Split(val)
		if err != nil {
			return fmt.Errorf("invalid %s: %v", key, err)
		}
		if len(args) == 0 {
			return fmt.Errorf("%s entry cannot contain only space", key)
		}
		if !filepath.IsAbs(args[0]) && args[0] != filepath.Base(args[0]) {
			return fmt.Errorf("%s entry is relative; must be absolute path: %q", key, args[0])
		}
	}

	if !utf8.ValidString(val) {
		return fmt.Errorf("invalid UTF-8 in %s=... value", key)
	}
	if strings.Contains(val, "\x00") {
		return fmt.Errorf("invalid NUL in %s=... value", key)
	}
	if strings.ContainsAny(val, "\v\r\n") {
		return fmt.Errorf("invalid newline in %s=... value", key)
	}
	return nil
}

func readEnvFileLines(mustExist bool) []string {
	file, _, err := cfg.EnvFile()
	if file == "" {
		if mustExist {
			base.Fatalf("go: cannot find go env config: %v", err)
		}
		return nil
	}
	data, err := os.ReadFile(file)
	if err != nil && (!os.IsNotExist(err) || mustExist) {
		base.Fatalf("go: reading go env config: %v", err)

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