diff --git a/lib/typeprof/core/ast/sig_type.rb b/lib/typeprof/core/ast/sig_type.rb
index f48da01d..c400d916 100644
--- a/lib/typeprof/core/ast/sig_type.rb
+++ b/lib/typeprof/core/ast/sig_type.rb
@@ -826,7 +826,7 @@ def contravariant_vertex0(genv, changes, vtx, subst)
       end
 
       def typecheck(genv, changes, vtx, subst)
-        changes.add_edge(genv, vtx, subst[@var]) unless vtx == subst[@var]
+        changes.add_edge(genv, vtx.new_vertex(genv, self), subst[@var]) unless vtx == subst[@var]
         true
       end
 
diff --git a/scenario/regressions/hash-merge-bang.rb b/scenario/regressions/hash-merge-bang.rb
new file mode 100644
index 00000000..364f66f3
--- /dev/null
+++ b/scenario/regressions/hash-merge-bang.rb
@@ -0,0 +1,22 @@
+## update
+def option
+  { a: 1 }
+end
+
+def foo1
+  option.merge!(bar)
+end
+
+def foo2
+  option.merge!(bar)
+end
+
+def bar = {}
+
+## assert
+class Object
+  def option: -> Hash[:a, Integer]
+  def foo1: -> Hash[:a, Integer]
+  def foo2: -> Hash[:a, Integer]
+  def bar: -> Hash[untyped, untyped]
+end