diff --git a/setup.py b/setup.py
index 0f213a9d..28ce0b0d 100644
--- a/setup.py
+++ b/setup.py
@@ -46,5 +46,6 @@
                              sources = ["bitarray/_bitarray.c"]),
                    Extension(name = "bitarray._util",
                              sources = ["bitarray/_util.c"])],
+    zip_safe=False,  # see PEP 561
     **kwds
 )