diff --git a/openvpn-install.sh b/openvpn-install.sh
index 99e7bca..3fa2fe9 100755
--- a/openvpn-install.sh
+++ b/openvpn-install.sh
@@ -1036,16 +1036,18 @@ function revokeClient () {
 		exit 1
 	fi
 
-	echo ""
-	echo "Select the existing client certificate you want to revoke"
-	tail -n +2 /etc/openvpn/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | nl -s ') '
-	if [[ "$NUMBEROFCLIENTS" = '1' ]]; then
-		read -rp "Select one client [1]: " CLIENTNUMBER
-	else
-		read -rp "Select one client [1-$NUMBEROFCLIENTS]: " CLIENTNUMBER
-	fi
+	until [[ "$CLIENT" =~ ^[a-zA-Z0-9_]+$ ]]; do
+		echo ""
+		echo "Select the existing client certificate you want to revoke"
+		tail -n +2 /etc/openvpn/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | nl -s ') '
+		if [[ "$NUMBEROFCLIENTS" = '1' ]]; then
+			read -rp "Select one client [1]: " CLIENTNUMBER
+		else
+			read -rp "Select one client [1-$NUMBEROFCLIENTS]: " CLIENTNUMBER
+		fi
 
-	CLIENT=$(tail -n +2 /etc/openvpn/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | sed -n "$CLIENTNUMBER"p)
+		CLIENT=$(tail -n +2 /etc/openvpn/easy-rsa/pki/index.txt | grep "^V" | cut -d '=' -f 2 | sed -n "$CLIENTNUMBER"p)
+	done
 	cd /etc/openvpn/easy-rsa/
 	./easyrsa --batch revoke "$CLIENT"
 	EASYRSA_CRL_DAYS=3650 ./easyrsa gen-crl