custr.diff

diff --git a/usr/src/lib/libcustr/common/custr.c b/usr/src/lib/libcustr/common/custr.c
index 8c8288ab7d..110a2be1b6 100644
--- a/usr/src/lib/libcustr/common/custr.c
+++ b/usr/src/lib/libcustr/common/custr.c
@@ -110,10 +110,13 @@ custr_cstr(custr_t *cus)
 int
 custr_append_vprintf(custr_t *cus, const char *fmt, va_list ap)
 {
-       int len = vsnprintf(NULL, 0, fmt, ap);
+       va_list copy;
+       int len;
        size_t chunksz = STRING_CHUNK_SIZE;

-       if (len < 0) {
+       va_copy(copy, ap);
+
+       if ((len = vsnprintf(NULL, 0, fmt, ap)) < 0) {
                return (len);
        }

@@ -157,7 +160,7 @@ custr_append_vprintf(custr_t *cus, const char *fmt, va_list ap)
         * Append new string to existing string:
         */
        if ((len = vsnprintf(cus->cus_data + cus->cus_strlen,
-           cus->cus_datalen - cus->cus_strlen, fmt, ap)) < 0) {
+           cus->cus_datalen - cus->cus_strlen, fmt, copy)) < 0) {
                return (len);
        }
        cus->cus_strlen += len;