diff --git a/numad.c b/numad.c
index a6a7a5d..25363a2 100644
--- a/numad.c
+++ b/numad.c
@@ -153,7 +153,7 @@ void numad_log(int level, const char *fmt, ...) {
     *p++ = ' ';
     va_list ap;
     va_start(ap, fmt);
-    vsnprintf(p, BUF_SIZE, fmt, ap);
+    vsnprintf(p, BUF_SIZE - strlen(buf) , fmt, ap);
     va_end(ap);
     fprintf(log_fs, "%s", buf);
     fflush(log_fs);