diff --git a/beat/web/plotters/admin.py b/beat/web/plotters/admin.py
index 83d0dd6a3f9fbe8f02b24cf9c08e2a4f3c6f263e..d3f9cc85ca64f06e4761660de3b177ce18e75a5f 100644
--- a/beat/web/plotters/admin.py
+++ b/beat/web/plotters/admin.py
@@ -27,11 +27,10 @@
 
 '''Administrative add-on for the extended Django User model'''
 
-import six
-
 from django.contrib import admin
 from django import forms
 from django.core.files.base import ContentFile
+from django.utils import six
 
 from ..common.texts import Messages
 
@@ -134,7 +133,9 @@ class PlotterModelForm(forms.ModelForm):
 
         if 'declaration_file' in self.data and \
                 isinstance(self.data['declaration_file'], six.string_types):
-            self.data['declaration_file'] = ContentFile(self.data['declaration_file'], name='unsaved')
+            mutable_data = self.data.copy()
+            mutable_data['declaration_file'] = ContentFile(self.data['declaration_file'], name='unsaved')
+            self.data = mutable_data
 
 
 #----------------------------------------------------------