diff --git a/client.c b/client.c
index 82e43992..e1bd47c2 100644
--- a/client.c
+++ b/client.c
@@ -575,7 +575,7 @@ client_dispatch_wait(void *data0)
 			imsg_free(&imsg);
 			return (-1);
 		case MSG_SHELL:
-			if (data[datalen - 1] != '\0')
+			if (datalen == 0 || data[datalen - 1] != '\0')
 				fatalx("bad MSG_SHELL string");
 
 			clear_signals(0);
@@ -659,7 +659,7 @@ client_dispatch_attached(void)
 			kill(getpid(), SIGTSTP);
 			break;
 		case MSG_LOCK:
-			if (data[datalen - 1] != '\0')
+			if (datalen == 0 || data[datalen - 1] != '\0')
 				fatalx("bad MSG_LOCK string");
 
 			system(data);
diff --git a/server-client.c b/server-client.c
index a39f56d8..e202902e 100644
--- a/server-client.c
+++ b/server-client.c
@@ -961,12 +961,12 @@ server_client_msg_identify(struct client *c, struct imsg *imsg)
 		c->flags |= flags;
 		break;
 	case MSG_IDENTIFY_TERM:
-		if (data[datalen - 1] != '\0')
+		if (datalen == 0 || data[datalen - 1] != '\0')
 			fatalx("bad MSG_IDENTIFY_TERM string");
 		c->term = xstrdup(data);
 		break;
 	case MSG_IDENTIFY_TTYNAME:
-		if (data[datalen - 1] != '\0')
+		if (datalen == 0 || data[datalen - 1] != '\0')
 			fatalx("bad MSG_IDENTIFY_TTYNAME string");
 		c->ttyname = xstrdup(data);
 		break;
@@ -981,7 +981,7 @@ server_client_msg_identify(struct client *c, struct imsg *imsg)
 		c->fd = imsg->fd;
 		break;
 	case MSG_IDENTIFY_ENVIRON:
-		if (data[datalen - 1] != '\0')
+		if (datalen == 0 || data[datalen - 1] != '\0')
 			fatalx("bad MSG_IDENTIFY_ENVIRON string");
 		if (strchr(data, '=') != NULL)
 			environ_put(&c->environ, data);