diff --git a/fsl/utils/filetree/filetree.py b/fsl/utils/filetree/filetree.py
index c7d7bcf4d65e136e3f88504eff7ee23208722799..151583e30c9a88ec645eceb1bdc9868c642ff5f7 100644
--- a/fsl/utils/filetree/filetree.py
+++ b/fsl/utils/filetree/filetree.py
@@ -293,7 +293,7 @@ class FileTree(object):
 
         file_variables = {}
 
-        with open(filename, 'r') as f:
+        with open(str(filename), 'r') as f:
             for full_line in f:
                 # ignore anything behind the first #-character
                 line = full_line.split('#')[0]