diff --git a/test/py/Makefile.am b/test/py/Makefile.am
index f8a3aa0..d3f0a7d 100755
--- a/test/py/Makefile.am
+++ b/test/py/Makefile.am
@@ -39,6 +39,7 @@
         gen-py-dynamicslots/DebugProtoTest/__init__.py
 
 precross: $(THRIFT) $(thrift_gen)
+BUILT_SOURCES = $(thrift_gen)
 
 helper_scripts=                                 \
         TestClient.py                           \