diff --git a/lib/typeprof/core/ast/sig_type.rb b/lib/typeprof/core/ast/sig_type.rb
index cb7f358b6..20c0d4a0f 100644
--- a/lib/typeprof/core/ast/sig_type.rb
+++ b/lib/typeprof/core/ast/sig_type.rb
@@ -54,7 +54,7 @@ def attrs = { type_params:, block_required: }
 
     class SigTyNode < Node
       def covariant_vertex(genv, changes, subst)
-        vtx = changes.new_vertex(genv, self, subst)
+        vtx = changes.new_vertex(genv, self)
         covariant_vertex0(genv, changes, vtx, subst)
         vtx
       end
diff --git a/lib/typeprof/core/graph/site.rb b/lib/typeprof/core/graph/site.rb
index 31ea7f0d7..72ebfd2cb 100644
--- a/lib/typeprof/core/graph/site.rb
+++ b/lib/typeprof/core/graph/site.rb
@@ -19,9 +19,10 @@ def initialize(target)
       @new_depended_superclasses = []
     end
 
-    attr_reader :sites, :diagnostics, :covariant_types
+    attr_reader :sites, :diagnostics
 
-    def new_vertex(genv, sig_type_node, subst)
+    def new_vertex(genv, sig_type_node)
+      # This is used to avoid duplicated vertex generation for the same sig node
       @covariant_types[sig_type_node] ||= Vertex.new("rbs_type", sig_type_node)
     end
 
diff --git a/lib/typeprof/core/graph/vertex.rb b/lib/typeprof/core/graph/vertex.rb
index 9f6ae1ca8..c86ef5be5 100644
--- a/lib/typeprof/core/graph/vertex.rb
+++ b/lib/typeprof/core/graph/vertex.rb
@@ -99,8 +99,8 @@ def on_type_added(genv, src_var, added_types)
     def on_type_removed(genv, src_var, removed_types)
     end
 
-    def new_vertex(genv, show_name, node)
-      nvtx = Vertex.new(show_name, node)
+    def new_vertex(genv, show_name, origin)
+      nvtx = Vertex.new(show_name, origin)
       add_edge(genv, nvtx)
       nvtx
     end
@@ -191,8 +191,8 @@ def on_type_removed(genv, src_var, removed_types)
       end
     end
 
-    def new_vertex(genv, show_name, node)
-      nvtx = Vertex.new(show_name, node)
+    def new_vertex(genv, show_name, origin)
+      nvtx = Vertex.new(show_name, origin)
       add_edge(genv, nvtx)
       nvtx
     end