diff --git a/aclocal/ax_lib_zlib.m4 b/aclocal/ax_lib_zlib.m4
index 40e96df..64b0b85 100644
--- a/aclocal/ax_lib_zlib.m4
+++ b/aclocal/ax_lib_zlib.m4
@@ -111,9 +111,9 @@
           AC_ARG_WITH([zlib],
                       AS_HELP_STRING([--with-zlib@<:@=DIR@:>@], [use zlib (default is yes) - it is possible to specify an alternate root directory for zlib]),
                       [
-                       if test "$withval" = "xno"; then
+                       if test "x$withval" = "xno"; then
                          want_zlib="no"
-                       elif test "$withval" = "xyes"; then
+                       elif test "x$withval" = "xyes"; then
                          want_zlib="yes"
                          ax_zlib_path=""
                        else