diff --git a/crates/librqbit/src/chunk_tracker.rs b/crates/librqbit/src/chunk_tracker.rs
index d5bccbd..9e0893b 100644
--- a/crates/librqbit/src/chunk_tracker.rs
+++ b/crates/librqbit/src/chunk_tracker.rs
@@ -34,23 +34,32 @@ pub struct ChunkTracker {
 // E.g. we might have more pieces, but the client asks to download only some files
 // partially.
 fn compute_chunk_status(lengths: &Lengths, needed_pieces: &BF) -> anyhow::Result<BF> {
+    if needed_pieces.len() != lengths.total_pieces() as usize {
+        anyhow::bail!(
+            "bug: needed_pieces.len() != lengths.total_pieces(); {} != {}",
+            needed_pieces.len(),
+            lengths.total_pieces()
+        );
+    }
     let required_size = lengths.chunk_bitfield_bytes();
     let vec = vec![0u8; required_size];
     let mut chunk_bf = BF::from_boxed_slice(vec.into_boxed_slice());
     let range = 0..lengths.total_pieces() as usize;
     for piece_index in needed_pieces
         .get(range.clone())
-        .with_context(|| format!("error getting range {range:?} from needed_pieces"))?
+        .with_context(|| format!("bug: error getting range {range:?} from needed_pieces. needed_pieces.len() = {}, range={:?}", needed_pieces.len(), range))?
         .iter_zeros()
     {
         let offset = piece_index * lengths.default_chunks_per_piece() as usize;
-        let chunks_per_piece = lengths
-            .chunks_per_piece(lengths.try_validate_piece_index(piece_index as u32)?)
-            as usize;
+        let chunks_per_piece = lengths.chunks_per_piece(
+            lengths
+                .try_validate_piece_index(piece_index as u32)
+                .context("bug")?,
+        ) as usize;
         let range = offset..offset + chunks_per_piece;
         chunk_bf
             .get_mut(range.clone())
-            .with_context(|| format!("error getting range {range:?} from chunk_bf"))?
+            .with_context(|| format!("bug: error getting range {range:?} from chunk_bf"))?
             .fill(true);
     }
     Ok(chunk_bf)