diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index bcdd32a..6639721 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -1746,7 +1746,13 @@ rb_deflate_init_copy(VALUE self, VALUE orig)
 	raise_zlib_error(err, 0);
     }
     z1->input = NIL_P(z2->input) ? Qnil : rb_str_dup(z2->input);
-    z1->buf   = NIL_P(z2->buf)   ? Qnil : rb_str_dup(z2->buf);
+    z1->buf = Qnil;
+    if (!NIL_P(z2->buf)) {
+	/* next_out was copied from orig; point it into our own buffer */
+	z1->buf = rb_obj_hide(rb_str_new(RSTRING_PTR(z2->buf), RSTRING_LEN(z2->buf)));
+	z1->stream.next_out = (Bytef*)RSTRING_END(z1->buf);
+	z1->stream.avail_out = 0;
+    }
     z1->flags = z2->flags;
 
     return self;
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index e1ecbf8..f6d29d3 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -39,6 +39,17 @@ def test_dup
       assert_equal("foobaz", Zlib::Inflate.inflate(s2))
     end
 
+    def test_dup_with_pending_output
+      s = Random.new(0).bytes(50_000)
+      z1 = Zlib::Deflate.new
+      z1 << s
+      z2 = z1.dup
+      z1 << "bar"
+      z2 << "baz"
+      assert_equal(s + "bar", Zlib::Inflate.inflate(z1.finish))
+      assert_equal(s + "baz", Zlib::Inflate.inflate(z2.finish))
+    end
+
     def test_deflate
       s = Zlib::Deflate.deflate("foo")
       assert_equal("foo", Zlib::Inflate.inflate(s))