diff --git a/src/customize b/src/customize
index ef8cff0cd04764d0b01cf656c59d59b42b08a83c..7a70198849f903fd2591b52c5eb91068ba0b9541 100644
--- a/src/customize
+++ b/src/customize
@@ -174,7 +174,7 @@ if [ "$ans" = "y" ] ; then
    echo "and subsequent ones should be preceeded by -l"
    def="X11"
    readln "X11 library [$def]?" "$def"
-   echo >>Makefile.custom "X11_LIB= $ans"
+   echo >>Makefile.custom "X11_LIB= -l$ans"
 fi