diff --git a/core/archipelago/src/music/tags.rs b/core/archipelago/src/music/tags.rs
index 5997da21..ff415635 100644
--- a/core/archipelago/src/music/tags.rs
+++ b/core/archipelago/src/music/tags.rs
@@ -165,10 +165,16 @@ mod tests {
     /// 64kbps, 44100Hz, mono, padded — the exact byte pattern lofty's own
     /// test suite uses as a known-good frame header
     /// (`mpeg/header.rs::tests::rev_search_for_frame_header`). Its computed
-    /// frame length (samples * bitrate * 125 / sample_rate + padding) is
-    /// 210 bytes.
+    /// frame length matches lofty's `Header::read` exactly:
+    /// `samples * bitrate * 125 / sample_rate` truncates *before* the
+    /// padding byte is added — `1152 * 64 * 125 / 44100 = 208`, `+ 1`
+    /// padding = **209** bytes. The frames written below must be exactly
+    /// this long, because lofty confirms a frame sync by comparing the
+    /// candidate header against the bytes found exactly `len` later
+    /// (`find_next_frame`/`cmp_header`); an off-by-one frame length makes
+    /// the whole file "contain an invalid frame".
     const MP3_FRAME_HEADER: [u8; 4] = [0xFF, 0xFB, 0x52, 0xC4];
-    const MP3_FRAME_LEN: usize = 210;
+    const MP3_FRAME_LEN: usize = 209;
 
     fn mp3_frame() -> Vec<u8> {
         let mut frame = MP3_FRAME_HEADER.to_vec();