diff --git a/ostap/fitting/roostats.py b/ostap/fitting/roostats.py
index 7e0f9255..80e3be16 100644
--- a/ostap/fitting/roostats.py
+++ b/ostap/fitting/roostats.py
@@ -70,6 +70,17 @@
 ## allowed types for parameters 
 par_types = ROOT.RooAbsReal, 
 # ==============================================================================
+if ( 6 , 28 ) <= root_info : # =================================================
+    # ==========================================================================
+    def make_set ( source ) : return ROOT.RooArgSet ( source )
+    # ==========================================================================
+else : # =======================================================================
+    # ==========================================================================
+    def make_set ( source ) : 
+        result = ROOT.RooArgSet()
+        for item in source : result.add ( item )
+        return result
+# ==============================================================================
 ## @class ModelConfig
 #  Helper class to create `RooStats::ModelConfig`
 #  @see RooStats::ModelConfig 
@@ -192,8 +203,7 @@ def __init__  ( self                      ,
             constraints = cnts
             
             ## Inform ModelConfig on the constraints
-            if ( 6 , 28 ) <= root_info : 
-                self.__mc.SetExternalConstraints ( constraints )
+            if ( 6 , 28 ) <= root_info : self.__mc.SetExternalConstraints ( constraints )
             self.__external_constraints = constraints 
 
         self.__raw_pdf   =   raw_pdf 
@@ -218,21 +228,22 @@ def __init__  ( self                      ,
         assert all ( v in final_pdf for v in observables ), \
             "Specified observables are not consistent with PDF"
 
-        observables = ROOT.RooArgSet ( self.pdf_observable ( o , dataset ) for o in observables ) 
-        
+
+        observables = make_set ( self.pdf_observable ( o , dataset ) for o in observables ) 
+            
         ##  (6) set observables 
         self.__mc.SetObservables ( observables ) 
         self.__observables = observables
 
         ## (7) get parameters of interess
-        poi = ROOT.RooArgSet ( self.pdf_param ( p , dataset ) for p in poi ) 
+        poi = make_set ( self.pdf_param ( p , dataset ) for p in poi ) 
         mc.SetParametersOfInterest ( poi )
         self.__poi  = poi 
 
         ## (8) global observables
         assert all ( self.pdf_param ( p , dataset ) for p in global_observables )
-        if global_observables : 
-            global_observables = ROOT.RooArgSet ( v for v in global_observables )
+        if global_observables :
+            global_observables = make_set ( v for v in global_observables )                
             assert all ( any ( o in c for c in constraints ) for o in global_observables ) , \
                 "Global observables are inconsisent with external constraints!"
             mc.SetGlobalObservables  ( global_observables ) 
@@ -240,9 +251,9 @@ def __init__  ( self                      ,
 
         ## nuisancee = all_parameters - poi - global_observables 
         nuisance = final_pdf.getParameters ( dataset ) - poi - global_observables
-        if nuisance : ROOT.RooStats.RemoveConstantParameters ( nuisance ) 
-        ## (9) Nuisance parameters
+        if nuisance : ROOT.RooStats.RemoveConstantParameters ( nuisance )
         
+        ## (9) Nuisance parameters        
         mc.SetNuisanceParameters ( nuisance )
         self.__nuisance = nuisance 
         
@@ -256,7 +267,7 @@ def __init__  ( self                      ,
         ## (11) constrained parameters (not used by RooStats! 
         assert all ( self.pdf_param ( p , dataset ) for p in constrained  )
         if constrained :
-            constrained = ROOT.RooArgSet ( v for v in conditional )
+            constrained = make_set ( v for v in conditional )
             mc.SetConstraintParametrs ( constrained ) 
         self.__constrained = constrained  
 
diff --git a/ostap/fitting/tests/test_fitting_roostats2.py b/ostap/fitting/tests/test_fitting_roostats2.py
index 628dbb97..5309079c 100644
--- a/ostap/fitting/tests/test_fitting_roostats2.py
+++ b/ostap/fitting/tests/test_fitting_roostats2.py
@@ -375,7 +375,7 @@ def test_point_limit_hc  () :
 #  - Profile Likelihoood Calculator is used 
 def test_point_limit_pl () :
     """ Get the upper limit at given point for small signal at fixed mass
-    - resoltuion is fixed 
+    - resolution is fixed 
     - Profile-Likelihood Calculator is used 
     """
 
diff --git a/ostap/fitting/tests/test_fitting_roostats5.py b/ostap/fitting/tests/test_fitting_roostats5.py
index 5c7a414b..d90d8015 100644
--- a/ostap/fitting/tests/test_fitting_roostats5.py
+++ b/ostap/fitting/tests/test_fitting_roostats5.py
@@ -94,7 +94,7 @@
 model_sim.fitTo ( dataset , quiet = True ) 
 
 
-err       = 0.1 
+err        = 0.3
 reff       = ROOT.RooRealVar ( 'reff' , 'ratio of efficiencies' , 1.0 , 0 , 10 )
 ceff1      = model2.soft_constraint ( reff , VE ( 1.0, err**2 ) )