diff --git a/lib/rbs/environment.rb b/lib/rbs/environment.rb
index 0fb434714..777e46de3 100644
--- a/lib/rbs/environment.rb
+++ b/lib/rbs/environment.rb
@@ -58,12 +58,13 @@ def initialize
 
     def initialize_copy(other)
       @sources = other.sources.dup
-      @class_decls = other.class_decls.dup
+      @class_decls = other.class_decls.transform_values(&:dup)
       @interface_decls = other.interface_decls.dup
       @type_alias_decls = other.type_alias_decls.dup
       @constant_decls = other.constant_decls.dup
       @global_decls = other.global_decls.dup
       @class_alias_decls = other.class_alias_decls.dup
+      @normalize_module_name_cache = other.instance_variable_get(:@normalize_module_name_cache).dup
     end
 
     def self.from_loader(loader)
diff --git a/lib/rbs/environment/class_entry.rb b/lib/rbs/environment/class_entry.rb
index 73762330a..03fe326dc 100644
--- a/lib/rbs/environment/class_entry.rb
+++ b/lib/rbs/environment/class_entry.rb
@@ -12,6 +12,11 @@ def initialize(name)
         @context_decls = []
       end
 
+      def initialize_copy(other)
+        @context_decls = other.context_decls.dup
+        self
+      end
+
       def <<(context_decl)
         context_decls << context_decl
         @primary_decl = nil
diff --git a/lib/rbs/environment/module_entry.rb b/lib/rbs/environment/module_entry.rb
index 1f65cf490..47356c1b2 100644
--- a/lib/rbs/environment/module_entry.rb
+++ b/lib/rbs/environment/module_entry.rb
@@ -12,6 +12,11 @@ def initialize(name)
         @context_decls = []
       end
 
+      def initialize_copy(other)
+        @context_decls = other.context_decls.dup
+        self
+      end
+
       def <<(context_decl)
         context_decls << context_decl
         self