diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index 3efc166a..e9159e0f 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -109,10 +109,10 @@ our $perimeters_extrusion_width     = 0;
 our $infill_extrusion_width         = 0;
 our $bridge_flow_ratio              = 1;
 our $overlap_factor                 = 0.5;
-our $flow                           = Slic3r::Flow->new;
-our $first_layer_flow               = undef;
-our $perimeters_flow                = Slic3r::Flow->new;
-our $infill_flow                    = Slic3r::Flow->new;
+our $flow;
+our $first_layer_flow;
+our $perimeters_flow;
+our $infill_flow;
 
 # print options
 our $perimeters         = 3;
diff --git a/lib/Slic3r/Config.pm b/lib/Slic3r/Config.pm
index 0307566b..d7ea2369 100644
--- a/lib/Slic3r/Config.pm
+++ b/lib/Slic3r/Config.pm
@@ -672,13 +672,18 @@ sub validate {
         if $Slic3r::_first_layer_height > $Slic3r::nozzle_diameter;
     
     # calculate flow
+    $Slic3r::flow = Slic3r::Flow->new;
     $Slic3r::flow->calculate($Slic3r::extrusion_width);
     if ($Slic3r::first_layer_extrusion_width) {
         $Slic3r::first_layer_flow = Slic3r::Flow->new(layer_height => $Slic3r::_first_layer_height);
         $Slic3r::first_layer_flow->calculate($Slic3r::first_layer_extrusion_width);
     }
+    $Slic3r::perimeters_flow = Slic3r::Flow->new;
     $Slic3r::perimeters_flow->calculate($Slic3r::perimeters_extrusion_width || $Slic3r::extrusion_width);
+    
+    $Slic3r::infill_flow = Slic3r::Flow->new;
     $Slic3r::infill_flow->calculate($Slic3r::infill_extrusion_width || $Slic3r::extrusion_width);
+    
     Slic3r::debugf "Default flow width = %s, spacing = %s, min_spacing = %s\n",
         $Slic3r::flow->width, $Slic3r::flow->spacing, $Slic3r::flow->min_spacing;