diff --git a/lib/typeprof/core/env.rb b/lib/typeprof/core/env.rb
index 6ef3510b1..cbf4fe783 100644
--- a/lib/typeprof/core/env.rb
+++ b/lib/typeprof/core/env.rb
@@ -281,6 +281,9 @@ class Array[Elem]
         class Hash[K, V]
           include _Each[[K, V]]
         end
+        class Object
+          include Hash::_Key
+        end
       RBS
 
       # Loading frequently used modules first will reduces constant resolution
diff --git a/scenario/hash/key_check.rb b/scenario/hash/key_check.rb
new file mode 100644
index 000000000..802eb31fa
--- /dev/null
+++ b/scenario/hash/key_check.rb
@@ -0,0 +1,16 @@
+## update
+def test
+  h = { a: 1, b: 2 }
+  h.key?(:a)
+  h.key?("not a symbol")
+  h.key?(Object.new)
+end
+
+test
+
+## assert
+class Object
+  def test: -> bool
+end
+
+## diagnostics