diff --git a/tncattach.c b/tncattach.c
index ab5772c..04041eb 100644
--- a/tncattach.c
+++ b/tncattach.c
@@ -504,6 +504,10 @@ static error_t parse_opt(int key, char *arg, struct argp_state *state) {
 
             long prefixLen_l = strtol(prefixPart_s, NULL, 10); // TODO: Add handling here for errors (using errno)
 
+						if(prefixLen_l == 0) {
+								printf("Prefix length '%s' is not numeric\n", prefixPart_s);
+								exit(EXIT_FAILURE);
+						}
 
             arguments->ipv6 = ipPart_s;