diff --git a/sloth/core/commands.py b/sloth/core/commands.py
index 82e824e..ba95819 100644
--- a/sloth/core/commands.py
+++ b/sloth/core/commands.py
@@ -101,7 +101,8 @@ class AppendFilesCommand(BaseCommand):
         for filename in args[1:]:
             rel_filename = filename
             try:
-                rel_filename = os.path.relpath(filename, os.path.dirname(args[0]))
+                if not os.path.isabs(filename):
+                    rel_filename = os.path.relpath(filename, os.path.dirname(args[0]))
             except:
                 pass
 
@@ -117,6 +118,7 @@ class AppendFilesCommand(BaseCommand):
                 item.setUnlabeled(True)
         self.labeltool.saveAnnotations(args[0])
 
+
 class MergeFilesCommand(BaseCommand):
     """
     Merge annotations of two label files and create a new one from it.
@@ -202,6 +204,7 @@ class MergeFilesCommand(BaseCommand):
         
         return an1
 
+
 def _make_writeable(filename):
     """
     Make sure that the file is writeable. Useful if our source is