diff --git a/interface/lib/classes/tform.inc.php b/interface/lib/classes/tform.inc.php
index 7789a81e3677a997aca28fabe8e49f62470f9aec..66615d0eac14fb10c86f37eb664309d60b9e5913 100644
--- a/interface/lib/classes/tform.inc.php
+++ b/interface/lib/classes/tform.inc.php
@@ -799,7 +799,9 @@ class tform {
                                         }
                                 break;
                                 case 'UNIQUE':
-                                        if($this->action == 'NEW') {
+										if($validator['allowempty'] != 'y') $validator['allowempty'] = 'n';
+										if($validator['allowempty'] == 'n' || ($validator['allowempty'] == 'y' && $field_value != '')){
+											if($this->action == 'NEW') {
                                                 $num_rec = $app->db->queryOneRecord("SELECT count(*) as number FROM ".$escape.$this->formDef['db_table'].$escape. " WHERE $field_name = '".$app->db->quote($field_value)."'");
                                                 if($num_rec["number"] > 0) {
                                                         $errmsg = $validator['errmsg'];
@@ -809,7 +811,7 @@ class tform {
 															$this->errorMessage .= $errmsg."<br />\r\n";
 														}
                                                 }
-                                        } else {
+											} else {
                                                 $num_rec = $app->db->queryOneRecord("SELECT count(*) as number FROM ".$escape.$this->formDef['db_table'].$escape. " WHERE $field_name = '".$app->db->quote($field_value)."' AND ".$this->formDef['db_table_idx']." != ".$this->primary_id);
                                                 if($num_rec["number"] > 0) {
                                                         $errmsg = $validator['errmsg'];
@@ -819,7 +821,8 @@ class tform {
 															$this->errorMessage .= $errmsg."<br />\r\n";
 														}
                                                 }
-                                        }
+											}
+										}
                                 break;
                                 case 'NOTEMPTY':
                                         if(empty($field_value)) {
diff --git a/interface/web/client/form/client.tform.php b/interface/web/client/form/client.tform.php
index ad95413cd4678a925a1f7aade7e76ce91bd2b41d..1b66ac75407fbf27379fec2aa301162b95240533 100644
--- a/interface/web/client/form/client.tform.php
+++ b/interface/web/client/form/client.tform.php
@@ -118,6 +118,10 @@ $form["tabs"]['address'] = array (
 		'customer_no' => array (
 			'datatype'	=> 'VARCHAR',
 			'formtype'	=> 'TEXT',
+			'validators'	=> array ( 	0 => array (	'type'	=> 'UNIQUE',
+														'errmsg'=> 'customer_no_error_unique',
+														'allowempty' => 'y'),
+									),
 			'default'	=> '',
 			'value'		=> '',
 			'separator'	=> '',
diff --git a/interface/web/client/form/reseller.tform.php b/interface/web/client/form/reseller.tform.php
index 6163c7293a94cb9dabefb48f4500265349ecf642..c9079ec6a26212ade86c0492152ad995ddb7fd4d 100644
--- a/interface/web/client/form/reseller.tform.php
+++ b/interface/web/client/form/reseller.tform.php
@@ -118,6 +118,10 @@ $form["tabs"]['address'] = array (
 		'customer_no' => array (
 			'datatype'	=> 'VARCHAR',
 			'formtype'	=> 'TEXT',
+			'validators'	=> array ( 	0 => array (	'type'	=> 'UNIQUE',
+														'errmsg'=> 'customer_no_error_unique',
+														'allowempty' => 'y'),
+									),
 			'default'	=> '',
 			'value'		=> '',
 			'separator'	=> '',
diff --git a/interface/web/client/lib/lang/de_client.lng b/interface/web/client/lib/lang/de_client.lng
index 88ebdfecafa6de498783f303b376f57ed44787e9..d43c98a591e4511addc09de92cb6c47db6b40659 100644
--- a/interface/web/client/lib/lang/de_client.lng
+++ b/interface/web/client/lib/lang/de_client.lng
@@ -135,5 +135,6 @@ $wb['generate_password_txt'] = "Passwort erzeugen";
 $wb['repeat_password_txt'] = "Passwort wiederholen";
 $wb['password_mismatch_txt'] = "Die Passwörter stimmen nicht überein.";
 $wb['password_match_txt'] = "Die Passwörter stimmen überein.";
+$wb["customer_no_error_unique"] = 'Die Kundennummer muß einzigartig sein (oder leer).';
 ?>
 
diff --git a/interface/web/client/lib/lang/de_reseller.lng b/interface/web/client/lib/lang/de_reseller.lng
index 26e6c745ce3cb2634667f2609d845ee2b8b9ffe4..90d45e3cfc7cc889200b9d161b39d6deca07d51a 100644
--- a/interface/web/client/lib/lang/de_reseller.lng
+++ b/interface/web/client/lib/lang/de_reseller.lng
@@ -128,5 +128,6 @@ $wb['generate_password_txt'] = "Passwort erzeugen";
 $wb['repeat_password_txt'] = "Passwort wiederholen";
 $wb['password_mismatch_txt'] = "Die Passwörter stimmen nicht überein.";
 $wb['password_match_txt'] = "Die Passwörter stimmen überein.";
+$wb["customer_no_error_unique"] = 'Die Kundennummer muß einzigartig sein (oder leer).';
 ?>
 
diff --git a/interface/web/client/lib/lang/en_client.lng b/interface/web/client/lib/lang/en_client.lng
index 38946a48c06276ad505fcca1567d78ef666ea680..1334dc4ca20092796f97e649052f0cbfa15194db 100644
--- a/interface/web/client/lib/lang/en_client.lng
+++ b/interface/web/client/lib/lang/en_client.lng
@@ -139,4 +139,5 @@ $wb['repeat_password_txt'] = 'Repeat Password';
 $wb['password_mismatch_txt'] = 'The passwords do not match.';
 $wb['password_match_txt'] = 'The passwords do match.';
 $wb['email_error_isemail'] = 'Please enter a valid email address.';
+$wb["customer_no_error_unique"] = 'The customer no. must be unique (or empty).';
 ?>
diff --git a/interface/web/client/lib/lang/en_reseller.lng b/interface/web/client/lib/lang/en_reseller.lng
index bc26515501e779888aa9dbd88366f0c782326cbc..f23cd1896c1f362728c3e14900c652c53c26e94d 100644
--- a/interface/web/client/lib/lang/en_reseller.lng
+++ b/interface/web/client/lib/lang/en_reseller.lng
@@ -131,4 +131,5 @@ $wb['repeat_password_txt'] = 'Repeat Password';
 $wb['password_mismatch_txt'] = 'The passwords do not match.';
 $wb['password_match_txt'] = 'The passwords do match.';
 $wb['email_error_isemail'] = 'Please enter a valid email address.';
+$wb["customer_no_error_unique"] = 'The customer no. must be unique (or empty).';
 ?>