invalid value for ' ' in ' ': value should not have…

Error message

invalid value for '%s' in '%s': value should not have whitespace

What it means

Returned by MountOpt.Set (opts/mount.go:51) when an option value has leading or trailing whitespace. The parser trims the value and compares against the raw value; if they differ it rejects the field, since embedded padding around values is almost always a mistake and would silently corrupt paths/names.

Solutions

Trim spaces from values: source=/var/data (no padding).
Avoid `key = value` style; use key=value with no surrounding whitespace.
If whitespace inside the value is genuinely intended (e.g. a label), note this parser does not allow leading/trailing whitespace — sanitize first.
Regenerate compose/templated mount strings without spaces around '='.

Example fix

// before
--mount "type=volume,source= mydata ,target=/data"

// after
--mount "type=volume,source=mydata,target=/data"

Defensive patterns

Strategy: validation

Validate before calling

// rejectValueWhitespace ensures no value has surrounding whitespace.
func rejectValueWhitespace(spec string) error {
    r := csv.NewReader(strings.NewReader(spec))
    fields, err := r.Read()
    if err != nil {
        return err
    }
    for _, f := range fields {
        _, v, has := strings.Cut(f, "=")
        if has && v != strings.TrimSpace(v) {
            return fmt.Errorf("value %q has surrounding whitespace", v)
        }
    }
    return nil
}

Try / catch

if err := m.Set(spec); err != nil {
    return fmt.Errorf("mount %q: %w", spec, err)
}

Prevention

Generate mounts from structured fields; never string-format with padding.
Trim generated values explicitly before joining.
Add a unit test that asserts emitted specs round-trip through MountOpt.Set.

When it happens

Trigger: A --mount CSV field whose value has surrounding spaces or tabs, e.g. `source= foo`, `target=/data `, or a value embedded as ` source = /data `. Keys are checked separately (error 623); this is specifically the value side.

Common situations: Spaces around '=' for readability, tabs in generated YAML/CSV, or quoted shell values that retained padding.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/cb9a941580c3fe13. Report an issue: GitHub.

Appendix: source

Thrown at opts/mount.go:51

		return err
	}

	mount := mounttypes.Mount{
		Type: mounttypes.TypeVolume, // default to volume mounts
	}

	for _, field := range fields {
		key, val, hasValue := strings.Cut(field, "=")
		if k := strings.TrimSpace(key); k != key {
			return fmt.Errorf("invalid option '%s' in '%s': option should not have whitespace", k, field)
		}
		if hasValue {
			v := strings.TrimSpace(val)
			if v == "" {
				return fmt.Errorf("invalid value for '%s': value is empty", key)
			}
			if v != val {
				return fmt.Errorf("invalid value for '%s' in '%s': value should not have whitespace", key, field)
			}
		}

		// TODO(thaJeztah): these options should not be case-insensitive.
		key = strings.ToLower(key)

		if !hasValue {
			switch key {
			case "readonly", "ro", "volume-nocopy", "bind-nonrecursive", "bind-create-src":
				// boolean values
			default:
				return fmt.Errorf("invalid field '%s' must be a key=value pair", field)
			}
		}

		switch key {
		case "type":
			mount.Type = mounttypes.Type(strings.ToLower(val))

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