diff --git a/tiramisu/api.py b/tiramisu/api.py
index d788d10..df43aac 100644
--- a/tiramisu/api.py
+++ b/tiramisu/api.py
@@ -576,7 +576,16 @@ class TiramisuOptionValue(CommonTiramisuOption):
                                        self.config_bag)
 
     @count
-    def _len(self):
+    def _len_master(self):
+        """length of master option (only for slave option)"""
+        option = self._get_option()
+        # for example if index is None
+        if '_length' not in vars(self):
+            self._length = self.subconfig.cfgimpl_get_length()
+        return self._length
+
+    @count
+    def _len_slave(self):
         """length of slave option (only for slave option)"""
         option = self._get_option()
         # for example if index is None
@@ -589,8 +598,11 @@ class TiramisuOptionValue(CommonTiramisuOption):
             return self._list
         elif name == 'pop' and self._get_option().impl_is_master_slaves('master'):
             return self._pop
-        elif name == 'len' and self._get_option().impl_is_master_slaves('slave'):
-            return self._len
+        elif name == 'len':
+            if self._get_option().impl_is_master_slaves('slave'):
+                return self._len_slave
+            if self._get_option().impl_is_master_slaves('master'):
+                return self._len_master
         raise APIError(_('{} is unknown').format(name))
 
     @count