diff --git a/src/Pure/General/compress.scala b/src/Pure/General/compress.scala
--- a/src/Pure/General/compress.scala
+++ b/src/Pure/General/compress.scala
@@ -1,63 +1,67 @@
 /*  Title:      Pure/General/compress.scala
     Author:     Makarius
 
 Support for generic data compression.
 */
 
 package isabelle
 
 
 import org.tukaani.xz.{LZMA2Options, ArrayCache, BasicArrayCache}
 import com.github.luben.zstd.{BufferPool, NoPool, RecyclingBufferPool,
   ZstdInputStream, ZstdOutputStream}
 
 
 object Compress {
   /* options */
 
   object Options {
     def apply(): Options = Options_XZ()
   }
   sealed abstract class Options
   case class Options_XZ(level: Int = 3) extends Options {
     def make: LZMA2Options = {
       val opts = new LZMA2Options
       opts.setPreset(level)
       opts
     }
   }
   case class Options_Zstd(level: Int = 3) extends Options
 
 
   /* cache */
 
   class Cache private(val xz: ArrayCache, val zstd: BufferPool)
 
   object Cache {
     def none: Cache = { Zstd.init(); new Cache(ArrayCache.getDummyCache(), NoPool.INSTANCE) }
-    def make(): Cache = { Zstd.init(); new Cache(new BasicArrayCache, RecyclingBufferPool.INSTANCE) } // FIXME new pool
+    def make(): Cache = {
+      Zstd.init()
+      val pool = Untyped.constructor(classOf[RecyclingBufferPool]).newInstance()
+      new Cache(new BasicArrayCache, pool)
+    }
   }
 
 
   /* Scala functions in ML */
 
   object XZ_Compress extends Scala.Fun_Bytes("XZ.compress") {
     val here = Scala_Project.here
     def apply(arg: Bytes): Bytes = arg.compress(Options_XZ())
   }
 
   object XZ_Uncompress extends Scala.Fun_Bytes("XZ.uncompress") {
     val here = Scala_Project.here
     def apply(arg: Bytes): Bytes = arg.uncompress_xz()
   }
 
   object Zstd_Compress extends Scala.Fun_Bytes("Zstd.compress") {
     val here = Scala_Project.here
     def apply(arg: Bytes): Bytes = arg.compress(Options_Zstd())
   }
 
   object Zstd_Uncompress extends Scala.Fun_Bytes("Zstd.uncompress") {
     val here = Scala_Project.here
     def apply(arg: Bytes): Bytes = arg.uncompress_zstd()
   }
 }