diff --git a/vyper/builtins/_convert.py b/vyper/builtins/_convert.py
index 3fc12c6c20..546207072a 100644
--- a/vyper/builtins/_convert.py
+++ b/vyper/builtins/_convert.py
@@ -267,7 +267,7 @@ def _literal_decimal(expr, arg_typ, out_typ):
 def to_bool(expr, arg, out_typ):
     _check_bytes(expr, arg, out_typ, 32)  # should we restrict to Bytes[1]?
 
-    if isinstance(arg.typ, (BytesT, StringT)):
+    if isinstance(arg.typ, _BytestringT):
         # no clamp. checks for any nonzero bytes.
         arg = _bytes_to_num(arg, out_typ, signed=False)