diff --git a/lib/typeprof/core/env.rb b/lib/typeprof/core/env.rb
index f3d47eac..13a4a8e5 100644
--- a/lib/typeprof/core/env.rb
+++ b/lib/typeprof/core/env.rb
@@ -116,6 +116,10 @@ def get_instance_type(mod, type_args, changes, base_ty_env, base_ty)
         base_ty.mod.type_params.zip(base_ty.args) do |param, arg|
           ty_env[param] = arg || Source.new
         end
+      elsif base_ty.is_a?(Type::Singleton)
+        base_ty.mod.type_params&.each do |param|
+          ty_env[param] = Source.new
+        end
       end
       args = mod.type_params.zip(type_args).map do |param, arg|
         arg && changes ? arg.covariant_vertex(self, changes, ty_env) : Source.new
diff --git a/scenario/regressions/singleton-type-param.rb b/scenario/regressions/singleton-type-param.rb
new file mode 100644
index 00000000..9e202548
--- /dev/null
+++ b/scenario/regressions/singleton-type-param.rb
@@ -0,0 +1,7 @@
+## update
+# Passing class constants (Singleton types) to methods like `all?`
+# should not crash with "unknown type variable" error
+[].all?(Array)
+[].all?(Hash)
+
+## assert