diff --git a/lib/typeprof/core/ast/sig_type.rb b/lib/typeprof/core/ast/sig_type.rb
index c3b90632..57bbf76a 100644
--- a/lib/typeprof/core/ast/sig_type.rb
+++ b/lib/typeprof/core/ast/sig_type.rb
@@ -39,7 +39,7 @@ def self.typecheck_for_prepended_modules(genv, changes, a_ty, f_mod, f_args, sub
         if prep_decl.is_a?(AST::SigPrependNode) && prep_mod.type_params
           prep_ty = genv.get_instance_type(prep_mod, prep_decl.args, changes, {}, a_ty)
         else
-          type_params = prep_mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
+          type_params = prep_mod.type_params.map {|(_name, _default_ty)| genv.untyped_arg } # TODO: better support
           prep_ty = Type::Instance.new(genv, prep_mod, type_params)
         end
         if prep_ty.mod == f_mod
@@ -64,7 +64,7 @@ def self.typecheck_for_included_modules(genv, changes, a_ty, f_mod, f_args, subs
         if inc_decl.is_a?(AST::SigIncludeNode) && inc_mod.type_params
           inc_ty = genv.get_instance_type(inc_mod, inc_decl.args, changes, {}, a_ty)
         else
-          type_params = inc_mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
+          type_params = inc_mod.type_params.map {|(_name, _default_ty)| genv.untyped_arg } # TODO: better support
           inc_ty = Type::Instance.new(genv, inc_mod, type_params)
         end
         if inc_ty.mod == f_mod
@@ -89,7 +89,7 @@ def self.typecheck_for_extended_modules(genv, changes, a_ty, f_mod, f_args, subs
         if ext_decl.is_a?(AST::SigExtendNode) && ext_mod.type_params
           ext_ty = genv.get_instance_type(ext_mod, ext_decl.args, changes, {}, a_ty)
         else
-          type_params = ext_mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
+          type_params = ext_mod.type_params.map {|(_name, _default_ty)| genv.untyped_arg } # TODO: better support
           ext_ty = Type::Instance.new(genv, ext_mod, type_params)
         end
         if ext_ty.mod == f_mod
@@ -705,6 +705,10 @@ def covariant_vertex0(genv, changes, vtx, subst)
         return unless cpath
         mod = genv.resolve_cpath(cpath)
         args = @args.map {|arg| arg.covariant_vertex(genv, changes, subst) }
+        # Omitted type arguments (e.g., `Foo` for `Foo[T]`) default to untyped
+        mod.type_params.drop(args.size).each do |_, default_ty|
+          args << (default_ty ? default_ty.covariant_vertex(genv, changes, subst) : genv.untyped_arg)
+        end
         changes.add_edge(genv, Source.new(Type::Instance.new(genv, mod, args)), vtx)
       end
 
@@ -716,7 +720,7 @@ def contravariant_vertex0(genv, changes, vtx, subst)
         # TODO: report error for wrong type arguments
         # TODO: support default type args
         args = mod.type_params.zip(@args).map do |_, arg|
-          arg ? arg.contravariant_vertex(genv, changes, subst) : Source.new
+          arg ? arg.contravariant_vertex(genv, changes, subst) : genv.untyped_arg
         end
         changes.add_edge(genv, Source.new(Type::Instance.new(genv, mod, args)), vtx)
       end
@@ -1095,6 +1099,10 @@ def covariant_vertex0(genv, changes, vtx, subst)
         return unless cpath
         mod = genv.resolve_cpath(cpath)
         args = @args.map {|arg| arg.covariant_vertex(genv, changes, subst) }
+        # Omitted type arguments (e.g., `Foo` for `Foo[T]`) default to untyped
+        mod.type_params.drop(args.size).each do |_, default_ty|
+          args << (default_ty ? default_ty.covariant_vertex(genv, changes, subst) : genv.untyped_arg)
+        end
         changes.add_edge(genv, Source.new(Type::Instance.new(genv, mod, args)), vtx)
       end
 
@@ -1104,6 +1112,7 @@ def contravariant_vertex0(genv, changes, vtx, subst)
         return unless cpath
         mod = genv.resolve_cpath(cpath)
         args = @args.map {|arg| arg.contravariant_vertex(genv, changes, subst) }
+        mod.type_params.drop(args.size).each { args << genv.untyped_arg }
         changes.add_edge(genv, Source.new(Type::Instance.new(genv, mod, args)), vtx)
       end
 
diff --git a/lib/typeprof/core/env.rb b/lib/typeprof/core/env.rb
index d1b8ce83..68c31038 100644
--- a/lib/typeprof/core/env.rb
+++ b/lib/typeprof/core/env.rb
@@ -42,6 +42,11 @@ def initialize
 
       @bot_type = Type::Bot.new(self)
 
+      # The vertex for a type argument that is not given. This must be shared:
+      # a fresh vertex makes a different Instance type and a different edge on
+      # every run, so a box that uses it is re-run forever.
+      @untyped_arg = Source.new
+
       @run_count = 0
     end
 
@@ -52,7 +57,7 @@ def initialize
     attr_reader :obj_type, :nil_type, :true_type, :false_type, :str_type
     attr_reader :int_type, :float_type, :rational_type, :complex_type
     attr_reader :proc_type, :symbol_type, :method_type, :set_type, :regexp_type
-    attr_reader :bot_type
+    attr_reader :bot_type, :untyped_arg
 
     def gen_ary_type(elem_vtx)
       Type::Instance.new(self, @mod_ary, [elem_vtx])
@@ -130,18 +135,18 @@ def get_instance_type(mod, type_args, changes, base_ty_env, base_ty)
       ty_env = base_ty_env.dup
       if base_ty.is_a?(Type::Instance)
         base_ty.mod.type_params.zip(base_ty.args) do |(param, default_ty), arg|
-          ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : Source.new)
+          ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : @untyped_arg)
         end
       elsif base_ty.is_a?(Type::Singleton)
         base_ty.mod.type_params&.each do |(param, default_ty)|
-          ty_env[param] = default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : Source.new
+          ty_env[param] = default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : @untyped_arg
         end
       end
       args = mod.type_params.zip(type_args).map do |(param, default_ty), arg|
         if changes
-          (arg || default_ty)&.covariant_vertex(self, changes, ty_env) || Source.new
+          (arg || default_ty)&.covariant_vertex(self, changes, ty_env) || @untyped_arg
         else
-          Source.new
+          @untyped_arg
         end
       end
       Type::Instance.new(self, mod, args)
@@ -463,7 +468,7 @@ def get_self(genv)
       case @scope_level
       when :instance
         mod = genv.resolve_cpath(@cpath || [])
-        type_params = mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
+        type_params = mod.type_params.map {|(_name, _default_ty)| genv.untyped_arg } # TODO: better support
         ty = Type::Instance.new(genv, mod, type_params)
         Source.new(ty)
       when :class
diff --git a/lib/typeprof/core/graph/box.rb b/lib/typeprof/core/graph/box.rb
index 9883abc7..51ab2749 100644
--- a/lib/typeprof/core/graph/box.rb
+++ b/lib/typeprof/core/graph/box.rb
@@ -779,7 +779,7 @@ def run0(genv, changes)
         ty = Type::Singleton.new(genv, mod)
         param_map0 = Type.default_param_map(genv, ty)
       else
-        type_params = mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
+        type_params = mod.type_params.map {|(_name, _default_ty)| genv.untyped_arg } # TODO: better support
         ty = Type::Instance.new(genv, mod, type_params)
         param_map0 = Type.default_param_map(genv, ty)
         if ty.is_a?(Type::Instance)
@@ -984,7 +984,7 @@ def run0(genv, changes)
             ty_env = Type.default_param_map(genv, orig_ty)
             if ty.is_a?(Type::Instance)
               ty.mod.type_params.zip(ty.args) do |(param, default_ty), arg|
-                ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(genv, changes, ty_env) : Source.new)
+                ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(genv, changes, ty_env) : genv.untyped_arg)
               end
             end
             mdecl.resolve_overloads(changes, genv, @node, ty_env, a_args, @ret) do |method_type|
@@ -1095,7 +1095,7 @@ def resolve_prepended_modules(genv, changes, base_ty_env, ty, mid, &blk)
         if prep_decl.is_a?(AST::SigPrependNode) && prep_mod.type_params
           prep_ty = genv.get_instance_type(prep_mod, prep_decl.args, changes, base_ty_env, ty)
         else
-          type_params = prep_mod.type_params.map { Source.new() } # TODO: better support
+          type_params = prep_mod.type_params.map { genv.untyped_arg } # TODO: better support
           prep_ty = Type::Instance.new(genv, prep_mod, type_params)
         end
 
@@ -1150,7 +1150,7 @@ def resolve_included_modules(genv, changes, base_ty_env, ty, mid, &blk)
         if inc_decl.is_a?(AST::SigIncludeNode) && inc_mod.type_params
           inc_ty = genv.get_instance_type(inc_mod, inc_decl.args, changes, base_ty_env, ty)
         else
-          type_params = inc_mod.type_params.map { Source.new() } # TODO: better support
+          type_params = inc_mod.type_params.map { genv.untyped_arg } # TODO: better support
           inc_ty = Type::Instance.new(genv, inc_mod, type_params)
         end
 
@@ -1181,7 +1181,7 @@ def resolve_extended_modules(genv, changes, base_ty_env, ty, mid, &blk)
         if ext_decl.is_a?(AST::SigExtendNode) && ext_mod.type_params
           ext_ty = genv.get_instance_type(ext_mod, ext_decl.args, changes, base_ty_env, ty)
         else
-          type_params = ext_mod.type_params.map { Source.new() } # TODO: better support
+          type_params = ext_mod.type_params.map { genv.untyped_arg } # TODO: better support
           ext_ty = Type::Instance.new(genv, ext_mod, type_params)
         end
 
diff --git a/lib/typeprof/core/type.rb b/lib/typeprof/core/type.rb
index 7ddf8634..512fe38e 100644
--- a/lib/typeprof/core/type.rb
+++ b/lib/typeprof/core/type.rb
@@ -77,7 +77,7 @@ def show
 
       def get_instance_type(genv)
         params = @mod.type_params
-        Instance.new(genv, @mod, params ? params.map { Source.new } : []) # TODO: respect param_default_types
+        Instance.new(genv, @mod, params ? params.map { genv.untyped_arg } : []) # TODO: respect param_default_types
       end
     end
 
diff --git a/scenario/rbs/generic-superclass-without-args.rb b/scenario/rbs/generic-superclass-without-args.rb
new file mode 100644
index 00000000..19314f9c
--- /dev/null
+++ b/scenario/rbs/generic-superclass-without-args.rb
@@ -0,0 +1,23 @@
+## update: test.rbs
+class Gen[T]
+  def self.create: () -> instance
+end
+
+class Object
+  def accept: (Gen[Integer]) -> String
+end
+
+## update: test.rb
+class Sub < Gen
+end
+
+def test
+  accept(Sub.create)
+end
+
+## assert
+class Sub < Gen
+end
+class Object
+  def test: -> String
+end
diff --git a/scenario/rbs/omitted-type-args.rb b/scenario/rbs/omitted-type-args.rb
new file mode 100644
index 00000000..bcd278e9
--- /dev/null
+++ b/scenario/rbs/omitted-type-args.rb
@@ -0,0 +1,40 @@
+## update: test.rbs
+class Gen[T]
+end
+
+class D[T, U = String]
+  def u: () -> U
+end
+
+interface _I[T]
+  def get: () -> T
+end
+
+class Object
+  def gen: () -> Gen
+  def d: () -> D
+  def i: () -> _I
+  def accept_gen: (Gen[Integer]) -> String
+  def accept_d: (D[Integer, String]) -> String
+  def accept_i: (_I[Integer]) -> String
+end
+
+## update: test.rb
+def test_gen = accept_gen(gen)
+def test_d = accept_d(d)
+def test_i = accept_i(i)
+def raw_gen = gen
+def raw_d = d
+def raw_d_u = d.u
+def raw_i = i
+
+## assert
+class Object
+  def test_gen: -> String
+  def test_d: -> String
+  def test_i: -> String
+  def raw_gen: -> Gen[untyped]
+  def raw_d: -> D[untyped, String]
+  def raw_d_u: -> String
+  def raw_i: -> _I[untyped]
+end