diff --git a/lib/libdevstat/devstat.c b/lib/libdevstat/devstat.c
index 7db6b9986b42..86694ea4e2a0 100644
--- a/lib/libdevstat/devstat.c
+++ b/lib/libdevstat/devstat.c
@@ -269,7 +269,8 @@ devstat_checkversion(kvm_t *kd)
 			errlen = snprintf(tmpstr, sizeof(tmpstr), "\n");
 			strncat(devstat_errbuf, tmpstr,
 				DEVSTAT_ERRBUF_SIZE - buflen - 1);
-			buflen += errlen;
+			if (errlen > 0)
+				buflen += errlen;
 		}
 
 		errlen = snprintf(tmpstr, sizeof(tmpstr),
@@ -281,7 +282,8 @@ devstat_checkversion(kvm_t *kd)
 		if (version == -1) {
 			strncat(devstat_errbuf, tmpstr,
 				DEVSTAT_ERRBUF_SIZE - buflen - 1);
-			buflen += errlen;
+			if (errlen > 0)
+				buflen += errlen;
 		} else {
 			strncpy(devstat_errbuf, tmpstr, DEVSTAT_ERRBUF_SIZE);
 			devstat_errbuf[DEVSTAT_ERRBUF_SIZE - 1] = '\0';
diff --git a/lib/libfetch/ftp.c b/lib/libfetch/ftp.c
index f1ffbf2dda0a..127401920526 100644
--- a/lib/libfetch/ftp.c
+++ b/lib/libfetch/ftp.c
@@ -744,7 +744,8 @@ _ftp_authenticate(int cd, struct url *url, struct url *purl)
 	if (!pwd || !*pwd) {
 	    if ((logname = getlogin()) == 0)
 		logname = FTP_ANONYMOUS_USER;
-	    len = snprintf(pbuf, MAXLOGNAME + 1, "%s@", logname);
+	    if ((len = snprintf(pbuf, MAXLOGNAME + 1, "%s@", logname)) == -1)
+		len = 0;
 	    gethostname(pbuf + len, sizeof pbuf - len);
 	    pwd = pbuf;
 	}
diff --git a/libexec/rlogind/rlogind.c b/libexec/rlogind/rlogind.c
index f0df9dfd099c..2719d0962ee5 100644
--- a/libexec/rlogind/rlogind.c
+++ b/libexec/rlogind/rlogind.c
@@ -555,6 +555,8 @@ fatal(f, msg, syserr)
 		    msg, strerror(errno));
 	else
 		len = snprintf(bp, sizeof(buf), "rlogind: %s.\r\n", msg);
+	if (len == -1)
+		len = 0;
 	(void) write(f, buf, bp + len - buf);
 	exit(1);
 }