What causes "invalid value for '%s': value is empty" in docker/cli?

Supply a concrete value: source=/var/data.

invalid value for ' ': value is empty

Error message

invalid value for '%s': value is empty

What it means

Returned by MountOpt.Set (opts/mount.go:48) when an option is written as `key=` but the value (after TrimSpace) is empty. The parser requires every key=value option to carry a non-blank value; bare toggles must instead omit the '=' entirely (handled by the boolean-option branch).

Solutions

Supply a concrete value: source=/var/data.
If the option is a boolean toggle (readonly, ro, volume-nocopy, bind-create-src), drop the '=' entirely instead of writing readonly=.
Ensure shell/env variables used to fill values are non-empty at evaluation time.
Remove the field if it is not needed.

Example fix

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

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

Defensive patterns

Strategy: validation

Validate before calling

// requireNonEmpty checks key=value options carry a value before MountOpt.Set.
func requireNonEmpty(spec string) error {
    r := csv.NewReader(strings.NewReader(spec))
    fields, err := r.Read()
    if err != nil {
        return err
    }
    boolToggles := map[string]bool{"readonly": true, "ro": true, "volume-nocopy": true, "bind-nonrecursive": true, "bind-create-src": true}
    for _, f := range fields {
        k, v, has := strings.Cut(f, "=")
        if has && strings.TrimSpace(v) == "" && !boolToggles[strings.ToLower(strings.TrimSpace(k))] {
            return fmt.Errorf("option %q has empty value", k)
        }
    }
    return nil
}

Try / catch

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

Prevention

Default unset template variables to a sentinel and fail loudly instead of emitting key=.
For booleans, omit the '=' rather than writing toggle=.
Validate generated mount strings against the known option set.

When it happens

Trigger: Passing `source=`, `target=`, `type=`, `volume-driver=` etc. in a --mount CSV — any field that has '=' followed by nothing or only whitespace.

Common situations: Templating/compose generation that emits `key=$VAR` with VAR unset, a trailing `=` typo, or copy-paste where the value was dropped.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/a4642494368e6395. Report an issue: GitHub.

Appendix: source

Thrown at opts/mount.go:48

	csvReader := csv.NewReader(strings.NewReader(value))
	fields, err := csvReader.Read()
	if err != nil {
		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)
			}
		}

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