diff --git a/jsignature/fields.py b/jsignature/fields.py
index ee99b26..af31674 100644
--- a/jsignature/fields.py
+++ b/jsignature/fields.py
@@ -6,7 +6,8 @@ import json
 from django.db import models
 from django.core import validators
 from django.core.exceptions import ValidationError
-from .forms import JSignatureField as JSignatureFormField
+from .forms import (JSignatureField as JSignatureFormField,
+JSIGNATURE_EMPTY_VALUES)
 
 class JSignatureField(models.Field):
     """
@@ -23,7 +24,7 @@ class JSignatureField(models.Field):
         Validates that the input can be red as a JSON object. Returns a Python
         datetime.date object.
         """
-        if value in validators.EMPTY_VALUES:
+        if value in JSIGNATURE_EMPTY_VALUES:
             return None
         elif isinstance(value, list):
             return value
@@ -33,10 +34,7 @@ class JSignatureField(models.Field):
             raise ValidationError('Invalid JSON format.')
 
     def get_prep_value(self, value):
-        return self.to_python(value)
-
-    def get_db_prep_value(self, value, connection, prepared=False):
-        if value in validators.EMPTY_VALUES:
+        if value in JSIGNATURE_EMPTY_VALUES:
             return None
         elif isinstance(value, basestring):
             return value
diff --git a/jsignature/forms.py b/jsignature/forms.py
index 22243af..644db8a 100644
--- a/jsignature/forms.py
+++ b/jsignature/forms.py
@@ -8,6 +8,8 @@ from django.core import validators
 from django.core.exceptions import ValidationError
 from .widgets import JSignatureWidget
 
+JSIGNATURE_EMPTY_VALUES = validators.EMPTY_VALUES + ('[]', )
+
 class JSignatureField(Field):
     """
     A field handling a signature capture field with with jSignature
@@ -19,7 +21,7 @@ class JSignatureField(Field):
             Validates that the input can be red as a JSON object. Returns a Python
             datetime.date object.
             """
-            if value in validators.EMPTY_VALUES:
+            if value in JSIGNATURE_EMPTY_VALUES:
                 return None
             try:
                 return json.loads(value)
diff --git a/jsignature/widgets.py b/jsignature/widgets.py
index 67ccc8e..1c7274a 100644
--- a/jsignature/widgets.py
+++ b/jsignature/widgets.py
@@ -4,10 +4,14 @@
 """
 import json
 from django.forms.widgets import HiddenInput
+from django.core import validators
+from django.core.exceptions import ValidationError
 from django.utils.safestring import mark_safe
 from django.utils.translation import ugettext_lazy as _
 from jsignature.settings import JSIGNATURE_DEFAULT_CONFIG
 
+JSIGNATURE_EMPTY_VALUES = validators.EMPTY_VALUES + ('[]', )
+
 class JSignatureWidget(HiddenInput):
     """
     A widget handling a signature capture field with with jSignature
@@ -22,19 +26,45 @@ class JSignatureWidget(HiddenInput):
         # Store jSignature js config
         self.jsignature_attrs = jsignature_attrs or {}
 
-    def render(self, name, value, attrs=None):
-
-        # Build config
-        jsign_id = 'jsign_%s' % name
+    def build_jsignature_config(self):
+        """ Build javascript config for jSignature initialization.
+            It's a dict with for which default values come from settings
+            and can be overriden by jsignature_attrs, given at widget
+            instanciation time """
         jsignature_config = JSIGNATURE_DEFAULT_CONFIG.copy()
         jsignature_config.update(self.jsignature_attrs)
+        return jsignature_config
+
+    def build_jsignature_id(self, name):
+        """ Build HTML id for jsignature container.
+            It's important because it's used in javascript code """
+        return 'jsign_%s' % name
+
+    def prep_value(self, value):
+        """ Prepare value before effectively render widget """
+        if value in JSIGNATURE_EMPTY_VALUES:
+            return "[]"
+        elif isinstance(value, basestring):
+            return value
+        elif isinstance(value, list):
+            return json.dumps(value)
+        raise ValidationError('Invalid format.')
+
+    def render(self, name, value, attrs=None):
+        """ Render widget """
+        # Build config
+        jsign_id = self.build_jsignature_id(name)
+        jsignature_config = self.build_jsignature_config()
+
+        # Prepare value
+        value = self.prep_value(value)
 
         # Build output
         hidden_input = super(JSignatureWidget, self).render(name, value, attrs)
         div = u'<div id="%s" class="jsign-container"></div>' % jsign_id
         clr = u'<input type="button" value="%s" class="btn">' % _('Reset')
         js  = u'$("#%s").jSignature(%s);' % (jsign_id, json.dumps(jsignature_config))
-        js += u'$("#%s").jSignature("setData", %s,"native");' % (jsign_id, json.dumps(value))
+        js += u'$("#%s").jSignature("setData", %s,"native");' % (jsign_id, value)
         js  = u'<script type="text/javascript">%s</script>' % js
         out = u'<div class="jsign-wrapper">%s%s%s%s</div>' % (hidden_input, div, clr, js)