diff --git a/ChangeLog b/ChangeLog
index e47ce5da96..8e1d294317 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+Sun Oct  7 23:54:33 2012  CHIKANAGA Tomoyuki  <nagachika@ruby-lang.org>
+
+	* ext/zlib/zlib.c (zstream_run_func): don't call inflate() when
+	  z->stream.avail_in == 0. it return Z_BUF_ERROR.
+	  but deflate() could be called with z->stream->avail_in == 0 because
+	  it has hidden buffer in z->stream->state (opaque structure).
+	  fix for gem install error. [ruby-dev:46149] [Bug #7040]
+
 Mon Oct  8 23:55:41 2012  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* eval.c (rb_mod_refinements): new method Module#refinements.
diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 6135e8259d..8c3d84d4a3 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -990,6 +990,14 @@ zstream_run_func(void *ptr)
 	    break;
 	}
 
+	if (z->stream.avail_in == 0 && z->func == &inflate_funcs) {
+	    /* break here because inflate() return Z_BUF_ERROR when avail_in == 0. */
+	    /* but deflate() could be called with avail_in == 0 (there's hidden buffer
+	       in zstream->state) */
+	    z->flags |= ZSTREAM_FLAG_IN_STREAM;
+	    break;
+	}
+
 	if (args->stream_output) {
 	    state = (int)(VALUE)rb_thread_call_with_gvl(zstream_expand_buffer_protect,
 							(void *)z);