diff --git a/toy-impl/inference.py b/toy-impl/inference.py
index 9eab2e5..9be17ad 100644
--- a/toy-impl/inference.py
+++ b/toy-impl/inference.py
@@ -32,6 +32,8 @@ def find_subst(ctx: dict[str, Type],
                 else:
                     if a not in b.constraints:
                         return f"{b} cannot take value of {a}"
+            if a == b:
+                return sub
             if b in a.get_vars():
                 return "Recursive type is not supported"
             sub[b.name] = a
diff --git a/toy-impl/test_subst.py b/toy-impl/test_subst.py
index 575dc20..c852ec9 100644
--- a/toy-impl/test_subst.py
+++ b/toy-impl/test_subst.py
@@ -53,6 +53,7 @@ try_case(ListType(i32), ListType(i32), {})
 try_case(TupleType([X, X]), TupleType([X, Y]), {})
 try_case(TupleType([X, X]), TupleType([Y, Y]), {})
 try_case(TupleType([X, Y]), TupleType([X, X]), {})
+try_case(TupleType([X, X]), TupleType([X, X]), {})
 try_case(TupleType([X, Y]), X, {})
 try_case(TupleType([i32, Y]), X, {})