diff --git a/lib/bitmap.c b/lib/bitmap.c
index 26ebafa..2c9242e 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -469,6 +469,10 @@
 	/* current bit is 'cur', most recently seen range is [rbot, rtop] */
 	int cur, rbot, rtop;
 
+	if (buflen == 0)
+		return 0;
+	buf[0] = 0;
+
 	rbot = cur = find_first_bit(maskp, nmaskbits);
 	while (cur < nmaskbits) {
 		rtop = cur;