diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 4cae484937..7906e0fba3 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -882,7 +882,6 @@ zstream_detach_input(struct zstream *z)
 	rb_obj_reveal(dst, rb_cString);
     }
     z->input = Qnil;
-    rb_obj_reveal(dst, rb_cString);
     return dst;
 }