diff --git a/make/py-topdir.mk b/make/py-topdir.mk
index acbcb8aa..9b2ab591 100644
--- a/make/py-topdir.mk
+++ b/make/py-topdir.mk
@@ -8,7 +8,7 @@ MYPY_CONFIG_PATH             = $(shell $(JW_PKG_PY) --topdir-format relative pro
 MYPY_PATH_DIRECTIVE          = mypy_path = "$(MYPY_CONFIG_PATH)"
 
 ifndef PY_CHECK_ROOTS
-  PY_CHECK_ROOTS            += $(wildcard $(TOPDIR)/src $(TOPDIR)/tools)
+  PY_CHECK_ROOTS             = $(wildcard $(TOPDIR)/src $(TOPDIR)/tools)
 endif
 
 ifndef PY_CHECK_RUFF