diff --git a/lib/rbs/prototype/rb.rb b/lib/rbs/prototype/rb.rb
index 88f3a412d..97c9a6bfe 100644
--- a/lib/rbs/prototype/rb.rb
+++ b/lib/rbs/prototype/rb.rb
@@ -694,7 +694,7 @@ def range_element_type(types)
 
         types = types.map do |t|
           if t.is_a?(Types::Literal)
-            type_name = TypeName.new(name: t.literal.class.name.to_sym, namespace: Namespace.root)
+            type_name = TypeName.new(name: t.literal.class.name&.to_sym || raise, namespace: Namespace.root)
             Types::ClassInstance.new(name: type_name, args: [], location: nil)
           else
             t
@@ -825,7 +825,7 @@ def sort_members!(decls)
           AST::Members::ClassVariable => -3,
           AST::Members::ClassInstanceVariable => -2,
           AST::Members::InstanceVariable => -1,
-        }
+        } #: Hash[Class, Integer]
         decls.sort_by! { |decl| [orders.fetch(decl.class, 0), i += 1] }
       end
     end
diff --git a/sig/types.rbs b/sig/types.rbs
index e9171300b..7b33abc90 100644
--- a/sig/types.rbs
+++ b/sig/types.rbs
@@ -301,16 +301,23 @@ module RBS
     end
 
     class Record
-      attr_reader all_fields: Hash[Symbol, [t, bool]]
+      type key = Symbol | String | Integer | bool
 
-      attr_reader fields: Hash[Symbol, t]
+      # All types of all files
+      #
+      # If the key is *required*, the second value of the tuple is `true`.
+      # If the key is *optional*, the second value of the tuple is `false`.
+      #
+      attr_reader all_fields: Hash[key, [t, bool]]
+
+      attr_reader fields: Hash[key, t]
 
-      attr_reader optional_fields: Hash[Symbol, t]
+      attr_reader optional_fields: Hash[key, t]
 
       type loc = Location[bot, bot]
 
-      def initialize: (fields: Hash[Symbol, t], location: loc?) -> void
-                    | (all_fields: Hash[Symbol, [t, bool]], location: loc?) -> void
+      def initialize: (fields: Hash[key, t], location: loc?) -> void
+                    | (all_fields: Hash[key, [t, bool]], location: loc?) -> void
 
       include _TypeBase