diff --git a/src/m4/ax_boost_base.m4 b/src/m4/ax_boost_base.m4
index c675f12d8..f941b27ec 100644
--- a/src/m4/ax_boost_base.m4
+++ b/src/m4/ax_boost_base.m4
@@ -155,7 +155,7 @@ if test "x$want_boost" = "xyes"; then
         AC_MSG_RESULT(yes)
     succeeded=yes
     found_system=yes
-        ],[
+        ],[:
         ])
     AC_LANG_POP([C++])
 
@@ -238,7 +238,7 @@ if test "x$want_boost" = "xyes"; then
             AC_MSG_RESULT(yes)
         succeeded=yes
         found_system=yes
-            ],[
+            ],[:
             ])
         AC_LANG_POP([C++])
     fi