--- debderiver
+++ debderiver_patch4.py
@@ -257,7 +257,7 @@
 
         # Reprepro wants some fields after others (e.g. Codename first). 
         # Sorting them all is the easiest way to do that. We take scalars 
-        #first, lists second, each alphabetically in so far as reprepro allows.
+        # first, lists second, each alphabetically in so far as reprepro allows.
         FIELD_ORDER_DISTS = ('Codename', 'Description', 'FakeComponentPrefix', 
                 'Label', 'Origin', 'SignWith', 'Suite', 'Update', 'Version', 
                 'Architectures', 'Components', 'DebIndices', 'DscIndices', 
@@ -435,6 +435,9 @@
     # Pour into objects.
     opsys = OperatingSystem(main_conf['os'], main_conf['description'], \
             main_conf['base_dir'])
+    if not os.access(opsys.base_dir, os.W_OK):
+        sys.stderr.write('"%s" is not a writable directory\n' % opsys.base_dir)
+        sys.exit(1)
     opsys.signing_key = str(main_conf['signing_key'])
 
     for dist in dists: