diff --git a/lib/typeprof/core/env/method_entity.rb b/lib/typeprof/core/env/method_entity.rb
index b5bfa98c..24734f03 100644
--- a/lib/typeprof/core/env/method_entity.rb
+++ b/lib/typeprof/core/env/method_entity.rb
@@ -2,60 +2,63 @@ module TypeProf::Core
   class MethodEntity
     def initialize
       @builtin = nil
-      @decls = Set.empty
-      @overloading_decls = Set.empty
-      @defs = Set.empty
-      @aliases = {}
-      @method_call_boxes = Set.empty
     end
 
-    attr_reader :decls, :defs, :aliases, :method_call_boxes
     attr_accessor :builtin
 
+    def decls = @decls ||= Set.empty
+    def defs = @defs ||= Set.empty
+    def aliases = @aliases ||= {}
+    def method_call_boxes = @method_call_boxes ||= Set.empty
+
+    private def overloading_decls = @overloading_decls ||= Set.empty
+
     def add_decl(decl)
       if decl.overloading
-        @overloading_decls << decl
+        overloading_decls << decl
       else
-        @decls << decl
+        decls << decl
       end
     end
 
     def remove_decl(decl)
       if decl.overloading
-        @overloading_decls.delete(decl) || raise
+        overloading_decls.delete(decl) || raise
       else
-        @decls.delete(decl) || raise
+        decls.delete(decl) || raise
       end
     end
 
     def add_def(mdef)
-      @defs << mdef
+      defs << mdef
       self
     end
 
     def remove_def(mdef)
-      @defs.delete(mdef) || raise
+      defs.delete(mdef) || raise
     end
 
     def add_alias(node, old_mid)
-      @aliases[node] = old_mid
+      aliases[node] = old_mid
     end
 
     def remove_alias(node)
-      @aliases.delete(node) || raise
+      aliases.delete(node) || raise
     end
 
     def exist?
-      @builtin || !@decls.empty? || !@defs.empty?
+      @builtin || (@decls && !@decls.empty?) || (@defs && !@defs.empty?)
     end
 
     def add_run_all_mdefs(genv)
+      return unless @defs
       @defs.each do |mdef|
         genv.add_run(mdef)
       end
     end
 
     def add_run_all_method_call_boxes(genv)
+      return unless @method_call_boxes
       @method_call_boxes.each do |box|
         genv.add_run(box)
       end
diff --git a/lib/typeprof/core/graph/vertex.rb b/lib/typeprof/core/graph/vertex.rb
index 747ccbf9..e1e5a299 100644
--- a/lib/typeprof/core/graph/vertex.rb
+++ b/lib/typeprof/core/graph/vertex.rb
@@ -2,7 +2,6 @@ module TypeProf::Core
   class BasicVertex
     def initialize(types)
       @types = types
-      @types_to_be_added = {}
     end
 
     attr_reader :types
@@ -10,13 +9,15 @@ def initialize(types)
     def each_type(&blk)
       @types.each_key(&blk)
 
-      until @types_to_be_added.empty?
-        h = @types_to_be_added.dup
-        h.each do |ty, source|
-          @types[ty] = source
+      if @types_to_be_added
+        until @types_to_be_added.empty?
+          h = @types_to_be_added.dup
+          h.each do |ty, source|
+            @types[ty] = source
+          end
+          @types_to_be_added.clear
+          h.each_key(&blk)
         end
-        @types_to_be_added.clear
-        h.each_key(&blk)
       end
     end
 
@@ -151,7 +152,7 @@ def on_type_added(genv, src_var, added_types)
           begin
             @types[ty] = set
           rescue
-            @types_to_be_added[ty] = set
+            (@types_to_be_added ||= {})[ty] = set
           end
           set << src_var
           (new_added_types ||= []) << ty
@@ -167,7 +168,7 @@ def on_type_added(genv, src_var, added_types)
     def on_type_removed(genv, src_var, removed_types)
       new_removed_types = nil
       removed_types.each do |ty|
-        raise "!!! not implemented" if @types_to_be_added[ty]
+        raise "!!! not implemented" if @types_to_be_added&.[](ty)
         @types[ty].delete(src_var) || raise
         if @types[ty].empty?
           @types.delete(ty) || raise