diff --git a/m4/ax_boost_thread.m4 b/m4/ax_boost_thread.m4
index 79e12cd..d1fa817 100644
--- a/m4/ax_boost_thread.m4
+++ b/m4/ax_boost_thread.m4
@@ -103,6 +103,7 @@ AC_DEFUN([AX_BOOST_THREAD],
                           break;
                           ;;
                         esac
+            ax_lib=
             if test "x$ax_boost_user_thread_lib" = "x"; then
                 for libextension in `ls -r $BOOSTLIBDIR/libboost_thread* 2>/dev/null | sed 's,.*/lib,,' | sed 's,\..*,,'`; do
                      ax_lib=${libextension}