diff --git a/lib/rbs/ast/members.rb b/lib/rbs/ast/members.rb
index 7888a1686..258a6df1f 100644
--- a/lib/rbs/ast/members.rb
+++ b/lib/rbs/ast/members.rb
@@ -30,6 +30,8 @@ def update(annotations: self.annotations, method_type: self.method_type)
           end
 
           def sub(subst)
+            return self if subst.empty?
+
             update(method_type: self.method_type.sub(subst))
           end
 
diff --git a/lib/rbs/definition.rb b/lib/rbs/definition.rb
index 67192f41b..0d697abe0 100644
--- a/lib/rbs/definition.rb
+++ b/lib/rbs/definition.rb
@@ -14,6 +14,8 @@ def initialize(parent_variable:, type:, declared_in:)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           parent_variable: parent_variable,
           type: type.sub(s),
@@ -142,6 +144,8 @@ def private?
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           super_method: super_method&.sub(s),
           defs: defs.map {|defn| defn.update(type: defn.type.sub(s)) },
@@ -347,6 +351,8 @@ def type_params_decl
     end
 
     def sub(s)
+      return self if s.empty?
+
       definition = self.class.new(type_name: type_name, self_type: _ = self_type.sub(s), ancestors: ancestors, entry: entry)
 
       definition.methods.merge!(methods.transform_values {|method| method.sub(s) })
diff --git a/lib/rbs/method_type.rb b/lib/rbs/method_type.rb
index 67d5ade3b..7ac56cd68 100644
--- a/lib/rbs/method_type.rb
+++ b/lib/rbs/method_type.rb
@@ -33,6 +33,8 @@ def to_json(state = _ = nil)
     def sub(s)
       sub = s.without(*type_param_names)
 
+      return self if sub.empty?
+
       self.class.new(
         type_params: type_params.map do |param|
           param.map_type do |bound|
diff --git a/lib/rbs/types.rb b/lib/rbs/types.rb
index 3ae75818f..42718e532 100644
--- a/lib/rbs/types.rb
+++ b/lib/rbs/types.rb
@@ -329,6 +329,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(name: name,
                        args: args.map {|ty| ty.sub(s) },
                        location: location)
@@ -371,6 +373,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(name: name,
                        args: args.map {|ty| ty.sub(s) },
                        location: location)
@@ -413,6 +417,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         Alias.new(name: name, args: args.map {|ty| ty.sub(s) }, location: location)
       end
 
@@ -469,6 +475,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -574,6 +582,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           all_fields: all_fields.transform_values {|ty, required| [ty.sub(s), required] },
           location: location
@@ -664,6 +674,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(type: type.sub(s), location: location)
       end
 
@@ -752,6 +764,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -841,6 +855,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -1093,6 +1109,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         map_type {|ty| ty.sub(s) }
       end
 
@@ -1272,6 +1290,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(subst)
+        return self if subst.empty?
+
         map_type { _1.sub(subst) }
       end
 
@@ -1346,6 +1366,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           type: type.sub(s),
           required: required,
@@ -1415,6 +1437,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           type: type.sub(s),
           block: block&.sub(s),