diff --git a/captcha/experimental.sh b/captcha/experimental.sh
index a69a60a..d87f15d 100755
--- a/captcha/experimental.sh
+++ b/captcha/experimental.sh
@@ -150,7 +150,7 @@ INSTRUCTIONS2="...en remplaçant chaque X par la somme"
 # ****************************************************************************
 
 MODULES=(CALCUL NOTIN REPLACE REVERSE)
-RMODULE=`seq 0 $((#MODULES[@]-1)) | sort -R | head -n 1`
+RMODULE=`seq 0 $(( (#MODULES[@]) - 1)) | sort -R | head -n 1`
 #eval "${MODULES[RMODULE]}"
 
 TEXT="$NEWINPUT"