diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index d9a5480da..33857fe00 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -147,7 +147,7 @@ ngx_vsnprintf(u_char *buf, size_t max, const char *fmt, va_list args)
             sign = 1;
             hexadecimal = 0;
             max_width = 0;
-            slen = 0;
+            slen = (size_t) -1;
 
             p = temp + NGX_INT64_LEN;
 
@@ -221,7 +221,7 @@ ngx_vsnprintf(u_char *buf, size_t max, const char *fmt, va_list args)
             case 's':
                 p = va_arg(args, u_char *);
 
-                if (slen == 0) {
+                if (slen == (size_t) -1) {
                     while (*p && buf < last) {
                         *buf++ = *p++;
                     }