diff --git a/ostap/fitting/printable.py b/ostap/fitting/printable.py
index 6e44bae0..42299570 100644
--- a/ostap/fitting/printable.py
+++ b/ostap/fitting/printable.py
@@ -38,7 +38,7 @@ def print_multiline ( o , content = 1 , verbose = False , indent = '' ) :
     - see RooPrintable 
     - see Ostap::Utils::print_printable1 
     """
-    if not valid_pointer ( o ) : return 'Invalid object'
+    if not valid_pointer ( o ) : return 'Invalid object/nullptr'
     return Ostap.Utils.print_printable1 ( o , content , verbose , indent )
 # =============================================================================
 ##  Use RooPrintable::printStream function
@@ -49,7 +49,7 @@ def print_stream  ( o , content = 1 , style = 3 , indent = '' ) :
     - see RooPrintable 
     - see Ostap::Utils::print_printable2
     """
-    if not valid_pointer ( o ) : return 'Invalid object'
+    if not valid_pointer ( o ) : return 'Invalid object/nullptr'
     return Ostap.Utils.print_printable2 ( o , content , style , indent )
 # =============================================================================
 ##  Use RooPrintable::printTree function
@@ -60,7 +60,7 @@ def print_tree ( o , indent = '' ) :
     - see RooPrintable 
     - see Ostap::Utils::print_printable_tree
     """
-    if not valid_pointer ( o ) : return 'Invalid object'
+    if not valid_pointer ( o ) : return 'Invalid object/nullptr'
     return Ostap.Utils.print_printable_tree ( o , indent )
 
 # =============================================================================
@@ -70,6 +70,7 @@ def _rp_print_ ( obj , opts = 'vv' , *style ) :
     >>> o = ...
     >>> print o 
     """
+    if not valid_pointer ( obj ) : return 'Invalid object/nullptr'
     return Ostap.Utils.print_printable ( obj , opts , *style )
 
 ROOT.RooPrintable.print_multiline = print_multiline
diff --git a/ostap/fitting/roocollections.py b/ostap/fitting/roocollections.py
index 62376cad..1cba1fb6 100644
--- a/ostap/fitting/roocollections.py
+++ b/ostap/fitting/roocollections.py
@@ -369,15 +369,17 @@ def _stl_contains_ ( lst , item ) :
         return any (  i.name == item  for i in lst )    
     return False 
 
-
-if ( 6 , 24 ) < root_info : _STLList = ROOT.RooSTLRefCountList[ROOT.RooAbsArg]
-else                      : _STLList = ROOT.RooSTLRefCountList(ROOT.RooAbsArg)
-
-_STLList .__str__      = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
-_STLList .__repr__     = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
-_STLList .__contains__ = _stl_contains_ 
-
-
+if ( 8 , 18 ) <= root_info < ( 6 , 24 ) : 
+    _STLList = ROOT.RooSTLRefCountList[ROOT.RooAbsArg]
+    _STLList .__str__      = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
+    _STLList .__repr__     = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
+    _STLList .__contains__ = _stl_contains_ 
+else ( 6 , 24 ) <= root_info : 
+    _STLList = ROOT.RooSTLRefCountList(ROOT.RooAbsArg)
+    _STLList .__str__      = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
+    _STLList .__repr__     = lambda s : str ( tuple ( _rs_list_ ( s ) ) ) if s else '[]'
+    _STLList .__contains__ = _stl_contains_ 
+    
 if not hasattr ( ROOT.RooArgList , '__iter__' ) or root_info < ( 6 , 31 ) : 
     ROOT.RooArgList . __iter__      = _ral_iter_
     _new_methods_ += [ ROOT.RooArgList. __iter__ ]
diff --git a/ostap/fitting/variables.py b/ostap/fitting/variables.py
index 0ac59d2e..044dd944 100644
--- a/ostap/fitting/variables.py
+++ b/ostap/fitting/variables.py
@@ -157,23 +157,44 @@ def _rrv_ve_ ( var ) :
     #
     return VE ( v , e2 )
 
-# ===============================================================================
-## does var depends on item ?
-#  - if  type(item) is RooAbsArg   : check var.dependsOn ( item )
-#  - if  type(item) is string_type : check item in var.servers() 
-#  - else , return False 
-def _var_contains_ ( var , item ) :
-    """ Does var depends on item ?
-    - if  type(item) is RooAbsArg   : check var.dependsOn ( item )
-    - if  type(item) is string_type : check item in var.servers() 
-    - else , return False 
-    """
-    if   isinstance ( var , ROOT.RooAbsArg ) : return var.dependsOn ( item ) 
-    elif isinstance ( var , string_types   ) : return item in var.servers() 
-    # 
-    return False 
-    
-# ==============================================================================
+
+# ==================================================================================
+if ( 6 , 18 ) <= root_info : # =====================================================
+    # ==============================================================================
+    ## does var depends on item ?
+    #  - if  type(item) is RooAbsArg   : check var.dependsOn ( item )
+    #  - if  type(item) is string_type : check item in var.servers() 
+    #  - else , return False
+    def _var_contains_ ( var , item ) :
+        """ Does var depends on item ?
+        - if  type(item) is RooAbsArg   : check var.dependsOn ( item )
+        - if  type(item) is string_type : check item in var.servers() 
+        - else , return False 
+        """
+        if   isinstance ( item , ROOT.RooAbsArg ) : return var.dependsOn ( item ) 
+        elif isinstance ( item , string_types   ) : return item in var.servers()
+        # 
+        return False
+    # ===============================================================================
+else : # ============================================================================
+    # ===============================================================================
+    ## does var depends on item ?
+    #  - if  type(item) is RooAbsArg   : check var.dependsOn  ( item )
+    #  - if  type(item) is string_type : check var.findServer ( item ) 
+    #  - else , return False
+    def _var_contains_ ( var , item ) :
+        """ Does var depends on item ?
+        - if  type(item) is RooAbsArg   : check var.dependsOn   ( item )
+        - if  type(item) is string_type : check var.findServer ( item )) 
+        - else , return False 
+        """
+        if   isinstance ( item , ROOT.RooAbsArg ) : return var.dependsOn ( item ) 
+        elif isinstance ( item , string_types   ) :
+            return valid_pointer ( var.findServer ( item ) ) 
+        # 
+        return False
+    # ==============================================================================
+# ==================================================================================
 ## check if the given value is in the range of RooRealVar
 #  @code 
 #  mass_range = ...