django - GeoDjango Polygon Field ParseException -

i using geodjango display form user can select area on map , name it.

i have following model

import django.contrib.gis.db.models models     class area(models.model):   name = models.charfield(max_length=25)   area_target = models.polygonfield(default='polygon empty') 

and form

from django.contrib.gis import forms class areaform(forms.form):   name = forms.charfield(max_length=160)   area_target = forms.polygonfield(srid=4326, required=false, widget=forms.osmwidget(attrs={'map_width': 600, 'map_height': 500})) 

the form displayed correctly , can select area using map widget, following error message when submit form:

geos_error: parseexception: expected 'z', 'm', 'zm', 'empty' or '(' encountered : ')'

geos_error: parseexception: expected 'z', 'm', 'zm', 'empty' or '(' encountered : ')'

error creating geometry value 'srid=900913;polygon()' (error encountered checking geometry returned geos c function "geoswktreader_read_r".)

this how polygon drawn:

as can see in debug output there no values edges of polygon.

i using django 1.9.4, postgres 9.5.1.0 db postgis extension , geos 3.5 geometry backend.

can tell me what's wrong?

polygon(()) invalid wkt, , throw parseexception shown.

try polygon empty create empty polygon geometry.

i'm not familiar geodjango, perhaps null/none option too.