diff --git a/configure.ac b/configure.ac
index f0ca114..325b300 100644
--- a/configure.ac
+++ b/configure.ac
@@ -522,8 +522,10 @@ AC_ARG_WITH([default-host], [AS_HELP_STRING([--with-default-host], [Set lib3270 
 	[ app_default_host="\"$withval\""],
 	[ app_default_host=""])
 
-AC_DEFINE_UNQUOTED(LIB3270_DEFAULT_HOST, $app_default_host, ["The default tn3270 host"])
-AC_SUBST(LIB3270_DEFAULT_HOST,$app_default_host)
+if test "$app_default_host" != ""; then
+	AC_DEFINE_UNQUOTED(LIB3270_DEFAULT_HOST, $app_default_host, ["The default tn3270 host"])
+	AC_SUBST(LIB3270_DEFAULT_HOST,$app_default_host)
+fi
 
 AC_ARG_ENABLE([static],
  	[AS_HELP_STRING([--enable-static], [Enable build and install the static library])],
--
libgit2 0.21.2