http://lists.gnu.org/archive/html/bug-bash/2014-11/msg00097.html

--- bash-4.3/builtins/mapfile.def
+++ bash-4.3/builtins/mapfile.def
@@ -339,7 +339,7 @@
   else
     array_name = list->word->word;
   
-  if (legal_identifier (array_name) == 0 && valid_array_reference (array_name) == 0)
+  if (legal_identifier (array_name) == 0)
     {
       sh_invalidid (array_name);
       return (EXECUTION_FAILURE);