package com.novoda.downloadmanager;

interface InternalDownloadFileStatus extends DownloadFileStatus {

    void update(FileSize fileSize, FilePath localFilePath);

    boolean isMarkedAsDownloading();

    boolean isMarkedAsQueued();

    boolean isMarkedForDeletion();

    void markAsDownloading();

    void isMarkedAsPaused();

    boolean isMarkedAsError();

    void markAsQueued();

    void markForDeletion();

    void markAsError(DownloadError.Error error);

    Optional<DownloadError> error();

}

package com.novoda.downloadmanager;

import android.support.annotation.Nullable;

interface InternalDownloadFileStatus extends DownloadFileStatus {

    void update(FileSize fileSize, FilePath localFilePath);

    boolean isMarkedAsDownloading();

    boolean isMarkedAsQueued();

    boolean isMarkedForDeletion();

    void markAsDownloading();

    void isMarkedAsPaused();

    boolean isMarkedAsError();

    void markAsQueued();

    void markForDeletion();

    void markAsError(DownloadError.Error error);

    @Nullable
    Optional<DownloadError> error();

}

--- 
+++ 
@@ -1,6 +1,4 @@
 package com.novoda.downloadmanager;
-
-import android.support.annotation.Nullable;
 
 interface InternalDownloadFileStatus extends DownloadFileStatus {
 
@@ -24,7 +22,6 @@
 
     void markAsError(DownloadError.Error error);
 
-    @Nullable
     Optional<DownloadError> error();
 
 }
Remove Nullable annotation on Optional.
