--- orig/packages/zlib/ZLibReadStream.st
+++ mod/packages/zlib/ZLibReadStream.st
@@ -183,7 +183,7 @@
 
     initializeZlibObject: level [
   <category: 'private zlib interface'>
-       self initializeZlibObject: level windowSize: -15
+       self initializeZlibObject: level windowSize: 15
     ]
 
     destroyZlibObject [
@@ -336,7 +336,7 @@
 
     initializeZlibObject [
   <category: 'private zlib interface'>
-       self initializeZlibObject: -15
+       self initializeZlibObject: 15
     ]
 
     destroyZlibObject [