diff --git a/toy-impl/parse_stmt.py b/toy-impl/parse_stmt.py
index b987a73..864b21a 100644
--- a/toy-impl/parse_stmt.py
+++ b/toy-impl/parse_stmt.py
@@ -128,12 +128,12 @@ def parse_if_stmt(ctx: Context,
         t = parse_expr(ctx, sym_table, node.test.left.args[0])
         if not isinstance(t, TypeVariable) or len(t.constraints) < 2:
             raise CustomError(
-                'type guard only support basic type variables with constraints',
+                'type guard only support basic bounded type variables',
                 node.test)
         t1, _ = parse_type(ctx, node.test.comparators[0])
         if t1 not in t.constraints:
             raise CustomError(
-                f'{t1} is not in constraints of {t}',
+                f'{t1} is not a possible instance of {t}',
                 node.test)
         t2 = [v for v in t.constraints if v != t1]
         try: