diff --git a/mp_pack.c b/mp_pack.c
index 447f1fdf..984a524d 100644
--- a/mp_pack.c
+++ b/mp_pack.c
@@ -15,6 +15,10 @@ mp_err mp_pack(void *rop, size_t maxcount, size_t *written, mp_order order, size
 
    mp_int t;
 
+   if ((size == 0u) || (nails >= (size * 8u))) {
+      return MP_VAL;
+   }
+
    count = mp_pack_count(op, nails, size);
 
    if (count > maxcount) {
diff --git a/mp_pack_count.c b/mp_pack_count.c
index aa682ba6..48699e16 100644
--- a/mp_pack_count.c
+++ b/mp_pack_count.c
@@ -5,7 +5,11 @@
 
 size_t mp_pack_count(const mp_int *a, size_t nails, size_t size)
 {
-   size_t bits = (size_t)mp_count_bits(a);
+   size_t bits;
+   if ((size == 0u) || (nails >= (size * 8u))) {
+      return 0u;
+   }
+   bits = (size_t)mp_count_bits(a);
    return ((bits / ((size * 8u) - nails)) + (((bits % ((size * 8u) - nails)) != 0u) ? 1u : 0u));
 }