docker/cli · error

type is required

Error message

type is required

What it means

Returned by validateExclusiveOptions (opts/mount_utils.go:49) when the mount's Type field is an empty string. When using the CLI's MountOpt.Set parser, Type defaults to mounttypes.TypeVolume, so this error only triggers when a mount.Mount struct is constructed directly (e.g., via the Go SDK/API client) without explicitly setting the Type field.

Solutions

1. Set the Type field on the mount.Mount struct to one of: mount.TypeBind, mount.TypeVolume, mount.TypeTmpfs, mount.TypeImage, mount.TypeCluster.
2. If building mounts from user input, default Type to mount.TypeVolume like the CLI parser does.

Example fix

// before: no type specified
mount := mount.Mount{
    Source: "/data",
    Target: "/data",
}

// after: set type explicitly
mount := mount.Mount{
    Type:   mount.TypeBind,
    Source: "/data",
    Target: "/data",
}

func validateMountType(m mounttypes.Mount) error {
    if m.Type == "" {
        return fmt.Errorf("mount type is required (one of: bind, volume, tmpfs, image, cluster)")
    }
    return nil
}

// Or default to volume like the CLI parser does:
func ensureMountType(m *mounttypes.Mount) {
    if m.Type == "" {
        m.Type = mounttypes.TypeVolume
    }
}

if err := validateMountOptions(&m); err != nil {
    if err.Error() == "type is required" {
        m.Type = mounttypes.TypeVolume // or TypeBind
        return validateMountOptions(&m)
    }
    return err
}

When it happens

Trigger: validateMountOptions is called on a mount.Mount struct where m.Type is "" (empty string). This occurs in programmatic/SDK usage where a mount is built without setting Type, not through the CLI --mount flag parser which always defaults Type to TypeVolume.

Common situations: Go code using the Docker client SDK to create a container with a HostConfig.Mounts entry that omits the Type field, or deserialized mount JSON lacking a type field.

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/b03c02ee2994a133. Report an issue: GitHub.

Appendix: source

Thrown at opts/mount_utils.go:49

				//	# no error
				return errors.New("option 'bind-recursive=readonly' requires 'bind-propagation=rprivate' to be specified in conjunction")
			}
		}
	}

	return nil
}

// validateExclusiveOptions checks if the given mount config only contains
// options for the given mount-type.
//
// This is the client-side equivalent of [mounts.validateExclusiveOptions] in
// the daemon, but with error-messages matching client-side flags / options.
//
// [mounts.validateExclusiveOptions]: https://github.com/moby/moby/blob/v2.0.0-beta.6/daemon/volume/mounts/validate.go#L31-L50
func validateExclusiveOptions(m *mount.Mount) error {
	if m.Type == "" {
		return errors.New("type is required")
	}

	if m.Type != mount.TypeBind && m.BindOptions != nil {
		return fmt.Errorf("cannot mix 'bind-*' options with mount type '%s'", m.Type)
	}
	if m.Type != mount.TypeVolume && m.VolumeOptions != nil {
		return fmt.Errorf("cannot mix 'volume-*' options with mount type '%s'", m.Type)
	}
	if m.Type != mount.TypeImage && m.ImageOptions != nil {
		return fmt.Errorf("cannot mix 'image-*' options with mount type '%s'", m.Type)
	}
	if m.Type != mount.TypeTmpfs && m.TmpfsOptions != nil {
		return fmt.Errorf("cannot mix 'tmpfs-*' options with mount type '%s'", m.Type)
	}
	if m.Type != mount.TypeCluster && m.ClusterOptions != nil {
		return fmt.Errorf("cannot mix 'cluster-*' options with mount type '%s'", m.Type)
	}
	return nil

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