diff --git a/internal/lz4block/blocks.go b/internal/lz4block/blocks.go
index 138083d9..a1bfa99e 100644
--- a/internal/lz4block/blocks.go
+++ b/internal/lz4block/blocks.go
@@ -8,9 +8,12 @@ const (
 	Block256Kb
 	Block1Mb
 	Block4Mb
-	Block8Mb = 2 * Block4Mb
 )
 
+// In legacy mode all blocks are compressed regardless
+// of the compressed size: use the bound size.
+var Block8Mb = uint32(CompressBlockBound(8 << 20))
+
 var (
 	BlockPool64K  = sync.Pool{New: func() interface{} { return make([]byte, Block64Kb) }}
 	BlockPool256K = sync.Pool{New: func() interface{} { return make([]byte, Block256Kb) }}
diff --git a/internal/lz4stream/block.go b/internal/lz4stream/block.go
index e9646546..459086f0 100644
--- a/internal/lz4stream/block.go
+++ b/internal/lz4stream/block.go
@@ -224,7 +224,9 @@ func (b *FrameDataBlock) Close(f *Frame) {
 func (b *FrameDataBlock) Compress(f *Frame, src []byte, level lz4block.CompressionLevel) *FrameDataBlock {
 	data := b.data
 	if f.isLegacy() {
-		data = data[:cap(data)]
+		// In legacy mode, the buffer is sized according to CompressBlockBound,
+		// but only 8Mb is buffered for compression.
+		src = src[:8<<20]
 	} else {
 		data = data[:len(src)] // trigger the incompressible flag in CompressBlock
 	}