diff --git a/pylot/core/io/default_parameters.py b/pylot/core/io/default_parameters.py
index b476a49b..f4648aae 100644
--- a/pylot/core/io/default_parameters.py
+++ b/pylot/core/io/default_parameters.py
@@ -452,9 +452,11 @@ defaults = {'rootpath': {'type': str,
                              'namestring': 'Wadati tolerance'},
 
             'jackfactor': {'type': float,
-                             'tooltip': 'pick is removed if the variance of the subgroup with the pick removed is larger than the mean variance of all subgroups times safety factor',
-                             'value': 5.0,
-                             'namestring': 'Jackknife safety factor'},
+                            'tooltip': 'pick is removed if the variance of the subgroup with the pick removed is larger than the mean variance of all subgroups times safety factor',
+                            'value': 5.0,
+                            'min:': 0.,
+                            'max': np.inf,
+                            'namestring': 'Jackknife safety factor'},
 
             'WAscaling': {'type': (float, float, float),
                           'tooltip': 'Scaling relation (log(Ao)+Alog(r)+Br+C) of Wood-Anderson amplitude Ao [nm] \