diff --git a/pySim/filesystem.py b/pySim/filesystem.py
index 2d9a07e0..67127555 100644
--- a/pySim/filesystem.py
+++ b/pySim/filesystem.py
@@ -661,7 +661,7 @@ class TransparentEF(CardEF):
                 filename = '%s/file' % dirname
                 # write existing data as JSON to file
                 with open(filename, 'w') as text_file:
-                    json.dump(orig_json, text_file, indent=4)
+                    json.dump(orig_json, text_file, indent=4, cls=JsonEncoder)
                 # run a text editor
                 self._cmd.run_editor(filename)
                 with open(filename, 'r') as text_file:
@@ -963,7 +963,7 @@ class LinFixedEF(CardEF):
                 filename = '%s/file' % dirname
                 # write existing data as JSON to file
                 with open(filename, 'w') as text_file:
-                    json.dump(orig_json, text_file, indent=4)
+                    json.dump(orig_json, text_file, indent=4, cls=JsonEncoder)
                 # run a text editor
                 self._cmd.run_editor(filename)
                 with open(filename, 'r') as text_file: