gistfile1.txt

diff --git a/src/org/jruby/ext/zlib/RubyZlib.java b/src/org/jruby/ext/zlib/RubyZlib.java
index a056cb8..65bae2f 100644
--- a/src/org/jruby/ext/zlib/RubyZlib.java
+++ b/src/org/jruby/ext/zlib/RubyZlib.java
@@ -1348,8 +1348,12 @@ public class RubyZlib {
         public IRubyObject rewind() {
             Ruby rt = getRuntime();
             // should invoke seek on realIo...
+            long n = io.getTotalIn();
+            if (io.getAvailIn() != null) {
+                n += io.getAvailIn().length;
+            }
             realIo.callMethod(rt.getCurrentContext(), "seek",
-                    new IRubyObject[]{rt.newFixnum(-io.getTotalIn()), rt.newFixnum(Stream.SEEK_CUR)});
+                    new IRubyObject[]{rt.newFixnum(-n), rt.newFixnum(Stream.SEEK_CUR)});
             // ... and then reinitialize
             initialize(realIo);
             return getRuntime().getNil();