diff --git a/whippet.m4 b/whippet.m4
index d92c36f9b..f364d1973 100644
--- a/whippet.m4
+++ b/whippet.m4
@@ -94,7 +94,7 @@ AC_DEFUN([WHIPPET_PKG_BDW],
   WHIPPET_BDW_GC=$bdw_gc])
 
 AC_DEFUN([WHIPPET_PKG_COLLECTOR],
- [AC_REQUIRE(WHIPPET_PKG_BDW)
+ [AC_REQUIRE([WHIPPET_PKG_BDW])
   PKG_CHECK_MODULES(WHIPPET_BDW, $WHIPPET_BDW_GC,
                     [whippet_have_bdw=yes], [whippet_have_bdw=no])
   AC_SUBST(WHIPPET_BDW_CFLAGS)