diff --git a/lib/libutil/expand_number.c b/lib/libutil/expand_number.c
--- a/lib/libutil/expand_number.c
+++ b/lib/libutil/expand_number.c
@@ -76,6 +76,8 @@
 		shift = 10;
 		break;
 	case 'b':
+		shift = 0;
+		break;
 	case '\0': /* No unit. */
 		*num = number;
 		return (0);
@@ -85,6 +87,16 @@
 		return (-1);
 	}
 
+	/*
+	 * endptr is the unit and there must be no
+	 * remaining characters.
+	 */
+	endptr++;
+	if (*endptr != '\0') {
+		errno = EINVAL;
+		return (-1);
+	}
+
 	if ((number << shift) >> shift != number) {
 		/* Overflow */
 		errno = ERANGE;
diff --git a/lib/libutil/tests/Makefile b/lib/libutil/tests/Makefile
--- a/lib/libutil/tests/Makefile
+++ b/lib/libutil/tests/Makefile
@@ -7,6 +7,7 @@
 TAP_TESTS_C+=	trimdomain_test
 TAP_TESTS_C+=	trimdomain-nodomain_test
 ATF_TESTS_C+=	cpuset_test
+ATF_TESTS_C+=	expand_number_test
 
 WARNS?=		2
 LIBADD+=	util
diff --git a/lib/libutil/tests/expand_number_test.c b/lib/libutil/tests/expand_number_test.c
new file mode 100644
--- /dev/null
+++ b/lib/libutil/tests/expand_number_test.c
@@ -0,0 +1,59 @@
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <errno.h>
+#include <libutil.h>
+
+#include <atf-c.h>
+
+ATF_TC_WITHOUT_HEAD(positivetests);
+ATF_TC_BODY(positivetests, tc)
+{
+	int retval;
+	uint64_t num;
+
+#define positive_tc(string, value) 							\
+	do {										\
+		ATF_CHECK_ERRNO(0, (retval = expand_number((string), &num)) == 0);	\
+		ATF_CHECK_EQ(retval, 0);						\
+		ATF_CHECK_EQ(num, (value));						\
+	} while (0)
+
+	positive_tc("123456", 123456);
+	positive_tc("123456b", 123456);
+	positive_tc("1k", 1024);
+	positive_tc("1K", 1024);
+	positive_tc("1m", 1048576);
+	positive_tc("1M", 1048576);
+	positive_tc("1g", 1073741824);
+	positive_tc("1G", 1073741824);
+	positive_tc("1t", 1099511627776);
+	positive_tc("1T", 1099511627776);
+	positive_tc("1p", 1125899906842624);
+	positive_tc("1P", 1125899906842624);
+	positive_tc("1e", 1152921504606846976);
+	positive_tc("1E", 1152921504606846976);
+	positive_tc("15E", 17293822569102704640ULL);
+}
+
+ATF_TC_WITHOUT_HEAD(negativetests);
+ATF_TC_BODY(negativetests, tc)
+{
+	int retval;
+	uint64_t num;
+
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("", &num));
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("x", &num));
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("1bb", &num));
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("1kb", &num));
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("1x", &num));
+	ATF_CHECK_ERRNO(EINVAL, retval = expand_number("1kx", &num));
+	ATF_CHECK_ERRNO(ERANGE, retval = expand_number("16E", &num));
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, positivetests);
+	ATF_TP_ADD_TC(tp, negativetests);
+	return (atf_no_error());
+}