Index: lnt/formats/JSONFormat.py
===================================================================
--- lnt/formats/JSONFormat.py
+++ lnt/formats/JSONFormat.py
@@ -19,9 +19,17 @@
     return json.load(path_or_file)
 
 
+def _dump_format(obj, fp):
+    # The json module produces str objects but fp is opened in binary mode
+    # (since Plistlib only dump to binary mode files) so we first dump into
+    # a string a convert to UTF-8 before outputing.
+    json_str = json.dumps(obj)
+    fp.write(json_str.encode())
+
+
 format = {
     'name': 'json',
     'predicate': _matches_format,
     'read': _load_format,
-    'write': json.dump,
+    'write': _dump_format,
 }