diff --git a/Makefile b/Makefile
index bf3a5200..17f29fb1 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 PROGS=notmuch-index-message xapian-dump
 
-MYCXXFLAGS=-Wall
+MYCXXFLAGS=-Wall -O0 -g
 
 all: $(PROGS)
 
diff --git a/notmuch-index-message.cc b/notmuch-index-message.cc
index 02f250b4..f16bd1ad 100644
--- a/notmuch-index-message.cc
+++ b/notmuch-index-message.cc
@@ -662,8 +662,10 @@ index_file (Xapian::WritableDatabase db,
     add_term (doc, "type", "mail");
     add_term (doc, "source_id", "1");
 
-    add_term (doc, "msgid", message_id);
-    doc.add_value (NOTMUCH_VALUE_MESSAGE_ID, message_id);
+    if (message_id) {
+	add_term (doc, "msgid", message_id);
+	doc.add_value (NOTMUCH_VALUE_MESSAGE_ID, message_id);
+    }
 
     if (thread_ids->len) {
 	unsigned int i;
@@ -687,7 +689,7 @@ index_file (Xapian::WritableDatabase db,
 	doc.add_value (NOTMUCH_VALUE_THREAD, thread_id->str);
 
 	g_string_free (thread_id, TRUE);
-    } else {
+    } else if (message_id) {
 	/* If not referenced thread, use the message ID */
 	add_term (doc, "thread", message_id);
 	doc.add_value (NOTMUCH_VALUE_THREAD, message_id);