diff --git a/wcs/fields.py b/wcs/fields.py
index 4de7f9db4..6856938c7 100644
--- a/wcs/fields.py
+++ b/wcs/fields.py
@@ -409,6 +409,16 @@ class Field(object):
                 self.display_locations.append('listings')
                 changed = True
                 self.in_listing = None
+        if six.PY3:
+            # repair dictionary attributes that may have been kept as bytes in
+            # the initial python 2 -> 3 conversion.
+            from wcs.qommon.storage import deep_bytes2str
+            for key in ('prefill', 'data_source'):
+                value = getattr(self, key, None)
+                if not value:
+                    continue
+                if b'type' in value:
+                    setattr(self, key, deep_bytes2str(getattr(self, key)))
         return changed
 
     def evaluate_condition(self, dict_vars, formdef, condition):