diff --git a/Makefile b/Makefile
index f70a8e4..86cc255 100644
--- a/Makefile
+++ b/Makefile
@@ -17,8 +17,8 @@
 P =	otpcli
 V =	0.0
 
-HDR =		err.h otp.h
-OBJ =		cli.o err.o otp.o
+HDR =		err.h mystrtonum.h otp.h
+OBJ =		cli.o err.o mystrtonum.o otp.o
 SRC =		${OBJ:.o=.c}
 DIST_FILES =	COPYING Makefile ${HDR} ${SRC}
 
diff --git a/cli.c b/cli.c
index 87e0215..bc264cd 100644
--- a/cli.c
+++ b/cli.c
@@ -22,6 +22,7 @@
 #include <unistd.h>
 
 #include "err.h"
+#include "mystrtonum.h"
 #include "otp.h"
 
 extern const char *__progname;
@@ -36,8 +37,7 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
-	char *end;
-	uintmax_t n;
+	const char *errstr;
 	uint64_t counter;
 	int32_t r;
 	int ch, do_hotp;
@@ -47,15 +47,9 @@ main(int argc, char *argv[])
 	while ((ch = getopt(argc, argv, "H:")) != -1) {
 		switch (ch) {
 		case 'H':
-			errno = 0;
-			n = strtoumax(optarg, &end, 0);
-			if (*optarg == '\0' || *end != '\0')
-				errx(1, "-H: not a number");
-			if (errno != 0)
-				err(1, "-H");
-			else if (n > UINT64_MAX)
-				errx(1, "-H: out of range");
-			counter = n;
+			counter = mystrtonum(optarg, 0, LLONG_MAX, &errstr);
+			if (errstr != NULL)
+				errx(1, "counter is %s: %s", errstr, optarg);
 			do_hotp = 1;
 			break;
 		default:
diff --git a/mystrtonum.c b/mystrtonum.c
new file mode 100644
index 0000000..3ad8e34
--- /dev/null
+++ b/mystrtonum.c
@@ -0,0 +1,67 @@
+/* This is OpenBSD's strtonum(3), renamed to mystrtonum */
+
+/*	$OpenBSD: strtonum.c,v 1.8 2015/09/13 08:31:48 guenther Exp $	*/
+
+/*
+ * Copyright (c) 2004 Ted Unangst and Todd Miller
+ * All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+
+#define	INVALID		1
+#define	TOOSMALL	2
+#define	TOOLARGE	3
+
+long long
+mystrtonum(const char *numstr, long long minval, long long maxval,
+    const char **errstrp)
+{
+	long long ll = 0;
+	int error = 0;
+	char *ep;
+	struct errval {
+		const char *errstr;
+		int err;
+	} ev[4] = {
+		{ NULL,		0 },
+		{ "invalid",	EINVAL },
+		{ "too small",	ERANGE },
+		{ "too large",	ERANGE },
+	};
+
+	ev[0].err = errno;
+	errno = 0;
+	if (minval > maxval) {
+		error = INVALID;
+	} else {
+		ll = strtoll(numstr, &ep, 10);
+		if (numstr == ep || *ep != '\0')
+			error = INVALID;
+		else if ((ll == LLONG_MIN && errno == ERANGE) || ll < minval)
+			error = TOOSMALL;
+		else if ((ll == LLONG_MAX && errno == ERANGE) || ll > maxval)
+			error = TOOLARGE;
+	}
+	if (errstrp != NULL)
+		*errstrp = ev[error].errstr;
+	errno = ev[error].err;
+	if (error)
+		ll = 0;
+
+	return (ll);
+}
diff --git a/mystrtonum.h b/mystrtonum.h
new file mode 100644
index 0000000..742100e
--- /dev/null
+++ b/mystrtonum.h
@@ -0,0 +1,16 @@
+/*
+ * libutil - C utility functions
+ *
+ * Written in 2020 by Lucas
+ *
+ * To the extent possible under law, the author(s) have dedicated all
+ * copyright and related and neighboring rights to this software to the
+ * public domain worldwide. This software is distributed without any
+ * warranty.
+ *
+ * You should have received a copy of the CC0 Public Domain Dedication
+ * along with this software. If not, see
+ * <http://creativecommons.org/publicdomain/zero/1.0/>.
+ */
+
+long long mystrtonum(const char *, long long, long long, const char **);