diff --git a/mbox2web b/mbox2web
index dbe1d8c..90160cd 100755
--- a/mbox2web
+++ b/mbox2web
@@ -1104,7 +1104,7 @@ class Thread:
                 for r in m.in_reply_to:
                     p = self.messages[r]
                     edges.append((p.x, p.y, m.x, m.y))
-            lines.append((m.date, m.mfrom, m.subject, m.encmsgid))
+            lines.append((m.date, m.mfrom or "(no sender)", m.subject or "(no subject)", m.encmsgid))
             y += 1
         s = "<table class='thread'>"
         s += "<tr>"