docker/cli · error

value is empty

Error message

value is empty

What it means

Returned by MountOpt.Set (opts/mount.go:27) when the value passed to a --mount flag, after strings.TrimSpace, is an empty string. This is the first validation check in the mount-option parser; an empty mount string cannot be parsed into any mount configuration.

Solutions

1. Ensure the --mount flag receives a non-empty value such as 'type=volume,source=myvol,target=/data'.
2. Check that shell variables used in --mount are set and non-empty before use.
3. Skip empty mount strings programmatically rather than passing them to Set.

Example fix

// before: empty mount value
// docker run --mount "" nginx

// after: valid mount spec
// docker run --mount type=volume,source=myvol,target=/data nginx

func validateMountValue(value string) error {
    if strings.TrimSpace(value) == "" {
        return fmt.Errorf("mount spec must not be empty")
    }
    return nil
}

if err := mountOpt.Set(value); err != nil {
    if err.Error() == "value is empty" {
        return fmt.Errorf("--mount requires a non-empty value, e.g., type=volume,source=vol,target=/data")
    }
    return err
}

When it happens

Trigger: MountOpt.Set("") or MountOpt.Set(" ") is called — the --mount flag receives an empty or whitespace-only value.

Common situations: Shell variable expansion to empty string in a --mount flag (e.g., --mount "$MOUNT_OPTS" where MOUNT_OPTS is unset), empty mount entry in a script or config, or a quoting error that produces an empty argument.

Related errors

• images options are incompatible with type volume
• tmpfs options are incompatible with type volume
• bind options are incompatible with type volume
• cluster options are incompatible with type volume
• invalid image source, source cannot be empty

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/ed273418a2dd8a26. Report an issue: GitHub.

Appendix: source

Thrown at opts/mount.go:27

	"strconv"
	"strings"

	"github.com/docker/go-units"
	mounttypes "github.com/moby/moby/api/types/mount"
)

// MountOpt is a Value type for parsing mounts
type MountOpt struct {
	values []mounttypes.Mount
}

// Set a new mount value
//
//nolint:gocyclo
func (m *MountOpt) Set(value string) error {
	value = strings.TrimSpace(value)
	if value == "" {
		return errors.New("value is empty")
	}

	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 {

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