diff --git a/plugins/genpass/genpass-xkcd b/plugins/genpass/genpass-xkcd
index a486ccb40..ed0397138 100755
--- a/plugins/genpass/genpass-xkcd
+++ b/plugins/genpass/genpass-xkcd
@@ -43,7 +43,7 @@ fi
 
 # Figure out how many words we need for 128 bits of security margin.
 # Each word adds log2($#words) bits.
-local -i n=$((ceil(128. / log2($#words))))
+local -i n=$((ceil(128. / (log($#words) / log(2)))))
 
 {
   local c