diff --git a/sys/contrib/libnv/bsd_nvpair.c b/sys/contrib/libnv/bsd_nvpair.c
--- a/sys/contrib/libnv/bsd_nvpair.c
+++ b/sys/contrib/libnv/bsd_nvpair.c
@@ -985,13 +985,13 @@
 	size = nvp->nvp_datasize;
 	tmp = (const char *)ptr;
 	for (ii = 0; ii < nvp->nvp_nitems; ii++) {
-		len = strnlen(tmp, size - 1) + 1;
-		size -= len;
-		if (tmp[len - 1] != '\0') {
+		if (size <= 0) {
 			ERRNO_SET(EINVAL);
 			return (NULL);
 		}
-		if (size < 0) {
+		len = strnlen(tmp, size - 1) + 1;
+		size -= len;
+		if (tmp[len - 1] != '\0') {
 			ERRNO_SET(EINVAL);
 			return (NULL);
 		}