diff --git a/configure.ac b/configure.ac
index ce273dd..82a6be4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -12,7 +12,7 @@ case $target_os in
      *openbsd*)
      ;;
      *)
-     CFLAGS=$CFLAGS -D__bounded__(x,y,z)
+     CFLAGS="$CFLAGS -D__bounded__(x,y,z)"
      ;;
 esac