diff --git a/intl/vasnprintf.c b/intl/vasnprintf.c
index 4a8e7f0..65ade71 100644
--- a/intl/vasnprintf.c
+++ b/intl/vasnprintf.c
@@ -758,7 +758,9 @@ convert_to_decimal (mpn_t a, size_t extra_zeroes)
size_t a_len = a.nlimbs;
/* 0.03345 is slightly larger than log(2)/(9*log(10)). */
size_t c_len = 9 * ((size_t)(a_len * (GMP_LIMB_BITS * 0.03345f)) + 1);
- char *c_ptr = (char *) malloc (xsum (c_len, extra_zeroes));
+ /* We need extra_zeroes bytes for zeroes, followed by c_len bytes for the
+ digits of a, followed by 1 byte for the terminating NUL. */
+ char *c_ptr = (char *) malloc (xsum (xsum (extra_zeroes, c_len), 1));
if (c_ptr != NULL)
{
char *d_ptr = c_ptr;