diff --git a/bzip2/src/main/scala/zio/compress/Bzip2BlockSize.scala b/bzip2/src/main/scala/zio/compress/Bzip2BlockSize.scala
index f01efeb..fc21403 100644
--- a/bzip2/src/main/scala/zio/compress/Bzip2BlockSize.scala
+++ b/bzip2/src/main/scala/zio/compress/Bzip2BlockSize.scala
@@ -5,7 +5,7 @@ package zio.compress
   * @param hundredKbIncrements
   *   a bzip2 block size in 100KB increments, valid values: 1 to 9
   */
-final case class Bzip2BlockSize(hundredKbIncrements: Int)
+final case class Bzip2BlockSize private (hundredKbIncrements: Int)
 
 object Bzip2BlockSize {
 
diff --git a/zstd/src/main/scala/zio/compress/ZstdCompressionLevel.scala b/zstd/src/main/scala/zio/compress/ZstdCompressionLevel.scala
index 1183bff..0d3ccde 100644
--- a/zstd/src/main/scala/zio/compress/ZstdCompressionLevel.scala
+++ b/zstd/src/main/scala/zio/compress/ZstdCompressionLevel.scala
@@ -56,3 +56,9 @@ object ZstdCompressionLevel {
   val VeryHighCompressionLevel: ZstdCompressionLevel = CompressionLevel19
   val UltraCompressionLevel: ZstdCompressionLevel = CompressionLevelUltra22
 }
+
+// The minimum level of -131072 was calculated as `-(1L << 17)`
+// and was derived from the C implementation of Zstd. See:
+// https://github.com/facebook/zstd/blob/b880f20d52a925ebee373b5050c206ba325d935d/lib/compress/zstd_compress.c#L6988,
+// https://github.com/facebook/zstd/blob/b880f20d52a925ebee373b5050c206ba325d935d/lib/zstd.h#L1249 and
+// https://github.com/facebook/zstd/blob/b880f20d52a925ebee373b5050c206ba325d935d/lib/zstd.h#L142-L143.