diff --git a/subst.c b/subst.c
--- a/subst.c
+++ b/subst.c
@@ -7962,7 +7962,7 @@ parameter_brace_transform (varname, value, ind, xform, rtype, quoted, pflags, fl
       return ((char *)NULL);
     }
 
-  if (valid_parameter_transform (xform) == 0)
+  if (xform[0] == 0 || valid_parameter_transform (xform) == 0)
     {
       this_command_name = oname;
 #if 0 /* TAG: bash-5.2 Martin Schulte <gnu@schrader-schulte.de> 10/2020 */
