diff --git a/lib/typeprof/core/ast.rb b/lib/typeprof/core/ast.rb
index 6264a095..46cbf927 100644
--- a/lib/typeprof/core/ast.rb
+++ b/lib/typeprof/core/ast.rb
@@ -11,7 +11,7 @@ def self.parse_rb(path, src)
       raise unless raw_scope.type == :program_node
 
       prism_source = result.source
-      file_context = FileContext.new(path, prism_source.code_units_cache(Encoding::UTF_16LE), result.comments)
+      file_context = FileContext.new(path, prism_source, result.comments)
 
       cref = CRef::Toplevel
       lenv = LocalEnv.new(file_context, cref, {}, [])
diff --git a/lib/typeprof/core/ast/base.rb b/lib/typeprof/core/ast/base.rb
index 915fa6be..cd1fbf8d 100644
--- a/lib/typeprof/core/ast/base.rb
+++ b/lib/typeprof/core/ast/base.rb
@@ -9,7 +9,7 @@ def initialize(raw_node, lenv)
         @ret = nil
 
         @changes = ChangeSet.new(self, nil)
-        @diagnostics = Set.empty
+        @diagnostics = Set::EMPTY
       end
 
       attr_reader :lenv
@@ -104,6 +104,7 @@ def install_copy(genv)
         @changes.reuse(self)
         (@prev_node || raise).diagnostics.each do |diag|
           diag.reuse(self)
+          @diagnostics = Set.empty if @diagnostics.equal?(Set::EMPTY)
           @diagnostics << diag
         end
         each_subnode do |subnode|
@@ -129,11 +130,12 @@ def narrowings
       end
 
       def add_diagnostic(diag)
+        @diagnostics = Set.empty if @diagnostics.equal?(Set::EMPTY)
         @diagnostics << diag
       end
 
       def remove_diagnostic(diag)
-        @diagnostics.delete(diag)
+        @diagnostics.delete(diag) unless @diagnostics.equal?(Set::EMPTY)
       end
 
       def diff(prev_node)
diff --git a/lib/typeprof/core/env.rb b/lib/typeprof/core/env.rb
index 41da1edf..6da8d0b2 100644
--- a/lib/typeprof/core/env.rb
+++ b/lib/typeprof/core/env.rb
@@ -305,12 +305,17 @@ class Hash[K, V]
   end
 
   class FileContext
-    attr_reader :path, :code_units_cache, :comments
-    def initialize(path, code_units_cache = nil, comments = nil)
+    attr_reader :path, :comments
+    def initialize(path, prism_source = nil, comments = nil)
       @path = path
-      @code_units_cache = code_units_cache
+      @prism_source = prism_source
+      @code_units_cache = nil
       @comments = comments
     end
+
+    def code_units_cache
+      @code_units_cache ||= @prism_source&.code_units_cache(Encoding::UTF_16LE)
+    end
   end
 
   class LocalEnv
diff --git a/lib/typeprof/core/util.rb b/lib/typeprof/core/util.rb
index 66fb3c82..7c73b385 100644
--- a/lib/typeprof/core/util.rb
+++ b/lib/typeprof/core/util.rb
@@ -81,5 +81,7 @@ def pretty_print(q)
       end
       q.text "]"
     end
+
+    EMPTY = empty.freeze
   end
 end