--- a/aclocal.m4
+++ b/aclocal.m4
@@ -978,16 +978,15 @@
   AC_ARG_ENABLE(readline,
     [  --enable-readline       use readline library (default is yes)],
     [if test "$enableval" = no; then
-       USE_READLINE=false;
-     fi], [])
+       USE_READLINE=false
+     fi])
   if $USE_READLINE; then
     AC_CHECK_LIB(readline, rl_set_keyboard_input_timeout, [
       LIBS="-lreadline $LIBS"
       AC_DEFINE(USE_READLINE, 1)
     ], [
-      USE_READLINE=false
-      warn_readline="To use GNU Readline, I need version 4.2 or later"
-      AC_MSG_WARN($warn_readline)
+      AC_MSG_WARN([I need GNU Readline 4.2 or later])
+      AC_MSG_ERROR([this is fatal unless you specify --disable-readline])
     ])
   fi
 ])