diff --git a/nbase/snprintf.c b/nbase/snprintf.c
index 102f1ff1a..0680786e5 100644
--- a/nbase/snprintf.c
+++ b/nbase/snprintf.c
@@ -639,7 +639,7 @@ vsnprintf (char *str, size_t sz, const char *format, va_list args)
   ret = xyzprintf (&state, format, args);
   *state.s = '\0';
   if (ret)
-    return sz;
+    return -1;
   else
     return state.s - state.str;
 }