diff --git a/mkvmuxer.cpp b/mkvmuxer.cpp
index 604d3f1..0f28315 100644
--- a/mkvmuxer.cpp
+++ b/mkvmuxer.cpp
@@ -2005,7 +2005,7 @@ bool Segment::AddCuePoint(uint64 timestamp, uint64 track) {
     return false;
 
   cue->set_time(timestamp / segment_info_.timecode_scale());
-  cue->set_block_number(cluster->blocks_added() + 1);
+  cue->set_block_number(cluster->blocks_added());
   cue->set_cluster_pos(cluster->position_for_cues());
   cue->set_track(track);
   if (!cues_.AddCue(cue))
diff --git a/mkvmuxer.hpp b/mkvmuxer.hpp
index 576556e..1a40b14 100644
--- a/mkvmuxer.hpp
+++ b/mkvmuxer.hpp
@@ -869,8 +869,9 @@ class Segment {
   Chapter* AddChapter();
 
   // Adds a cue point to the Cues element. |timestamp| is the time in
-  // nanoseconds of the cue's time. |track| is the Track of the Cue. Returns
-  // true on success.
+  // nanoseconds of the cue's time. |track| is the Track of the Cue. This
+  // function must be called after AddFrame to calculate the correct
+  // BlockNumber for the CuePoint. Returns true on success.
   bool AddCuePoint(uint64 timestamp, uint64 track);
 
   // Adds a frame to be output in the file. Returns true on success.