The ratio_over parameter was missing in the definition of first_layer_extrusion_width

lib/Slic3r/Config.pm

@@ -324,7 +324,7 @@ sub validate {
         my $max_nozzle_diameter = max(@{ $self->nozzle_diameter });
         die "Invalid extrusion width (too large)\n"
             if defined first { $_ > 10 * $max_nozzle_diameter }
-                map $self->get("${_}_extrusion_width"),
+                map $self->get_value("${_}_extrusion_width"),
                 qw(perimeter infill solid_infill top_infill support_material first_layer);
     }
     

xs/src/PrintConfig.hpp

@@ -285,9 +285,10 @@ class PrintConfigDef
 
         Options["first_layer_extrusion_width"].type = coFloatOrPercent;
         Options["first_layer_extrusion_width"].label = "First layer";
-        Options["first_layer_extrusion_width"].tooltip = "Set this to a non-zero value to set a manual extrusion width for first layer. You can use this to force fatter extrudates for better adhesion. If expressed as percentage (for example 120%) if will be computed over layer height.";
+        Options["first_layer_extrusion_width"].tooltip = "Set this to a non-zero value to set a manual extrusion width for first layer. You can use this to force fatter extrudates for better adhesion. If expressed as percentage (for example 120%) if will be computed over first layer height.";
         Options["first_layer_extrusion_width"].sidetext = "mm or % (leave 0 for default)";
         Options["first_layer_extrusion_width"].cli = "first-layer-extrusion-width=s";
+        Options["first_layer_extrusion_width"].ratio_over = "first_layer_height";
 
         Options["first_layer_height"].type = coFloatOrPercent;
         Options["first_layer_height"].label = "First layer height";