diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index d3ff368..56839b1 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -233,7 +233,10 @@ ngx_vslprintf(u_char *buf, u_char *last, const char *fmt, va_list args)
                 v = va_arg(args, ngx_str_t *);
 
                 len = ngx_min(((size_t) (last - buf)), v->len);
-                buf = ngx_cpymem(buf, v->data, len);
+                /* valgrind falsepositive if len is 0 */
+                if (len) {
+                    buf = ngx_cpymem(buf, v->data, len);
+                }
                 fmt++;
 
                 continue;

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Content-Disposition: form-data; name="description"