diff --git a/lib/darr.c b/lib/darr.c
index 4f3bd9fb67ab..7dcc6d3fabff 100644
--- a/lib/darr.c
+++ b/lib/darr.c
@@ -77,7 +77,7 @@ char *__darr_in_vsprintf(char **sp, bool concat, const char *fmt, va_list ap)
 	else if ((size_t)len < darr_avail(*sp))
 		_darr_len(*sp) += len;
 	else {
-		darr_ensure_cap(*sp, darr_len(*sp) + (size_t)len);
+		darr_ensure_cap(*sp, darr_len(*sp) + (size_t)len + 1);
 		goto again;
 	}
 	return *sp;