diff --git a/markets/dagstore/piecereader.go b/markets/dagstore/piecereader.go
index b76c24cea..9b09c50cf 100644
--- a/markets/dagstore/piecereader.go
+++ b/markets/dagstore/piecereader.go
@@ -119,11 +119,18 @@ func (p *pieceReader) ReadAt(b []byte, off int64) (n int, err error) {
 
 	// check if we need to burn some bytes
 	if off > p.rAt {
-		if _, err := io.CopyN(io.Discard, p.r, p.rAt-off); err != nil {
+		n, err := io.CopyN(io.Discard, p.r, off-p.rAt)
+		p.rAt += n
+		if err != nil {
 			return 0, xerrors.Errorf("discarding read gap: %w", err)
 		}
 	}
 
+	// sanity check
+	if off != p.rAt {
+		return 0, xerrors.Errorf("bad reader offset; requested %d; at %d", off, p.rAt)
+	}
+
 	// Read!
 	n, err = p.r.Read(b)
 	p.rAt += int64(n)