diff -up cups-1.4rc1/cgi-bin/admin.c.str3312 cups-1.4rc1/cgi-bin/admin.c
--- cups-1.4rc1/cgi-bin/admin.c.str3312	2009-05-11 22:21:29.000000000 +0100
+++ cups-1.4rc1/cgi-bin/admin.c	2009-08-27 16:33:00.691275487 +0100
@@ -628,6 +628,15 @@ do_am_class(http_t *http,		/* I - HTTP c
     return;
   }
 
+  if (!name)
+    {
+      cgiSetVariable("ERROR", cgiText(_("Missing form variable!")));
+      cgiStartHTML(title);
+      cgiCopyTemplateLang("error.tmpl");
+      cgiEndHTML();
+      return;
+    }
+
   for (ptr = name; *ptr; ptr ++)
     if ((*ptr >= 0 && *ptr <= ' ') || *ptr == 127 || *ptr == '/' || *ptr == '#')
       break;
@@ -662,8 +671,7 @@ do_am_class(http_t *http,		/* I - HTTP c
   request = ippNewRequest(CUPS_ADD_CLASS);
 
   httpAssembleURIf(HTTP_URI_CODING_ALL, uri, sizeof(uri), "ipp", NULL,
-                   "localhost", 0, "/classes/%s",
-		   cgiGetVariable("PRINTER_NAME"));
+                   "localhost", 0, "/classes/%s", name);
   ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_URI, "printer-uri",
                NULL, uri);
 
diff -up cups-1.4rc1/templates/modify-class.tmpl.str3312 cups-1.4rc1/templates/modify-class.tmpl
--- cups-1.4rc1/templates/modify-class.tmpl.str3312	2008-09-20 04:33:59.000000000 +0100
+++ cups-1.4rc1/templates/modify-class.tmpl	2009-08-27 16:33:00.693275103 +0100
@@ -4,6 +4,7 @@
 
 <FORM METHOD="POST" ACTION="/admin">
 <INPUT TYPE="HIDDEN" NAME="OP" VALUE="{op}">
+<INPUT TYPE="HIDDEN" NAME="PRINTER_NAME" VALUE="{printer_name}">
 
 <TABLE>
 <TR>