diff --git a/src/mxmake/tests/test_templates.py b/src/mxmake/tests/test_templates.py
index af89042..f04a2c2 100644
--- a/src/mxmake/tests/test_templates.py
+++ b/src/mxmake/tests/test_templates.py
@@ -665,7 +665,7 @@ def test_Makefile(self, tempdir):
                 # mxenv
                 ##############################################################################
 
-                export OS:=$(OS)
+                OS?=
 
                 # Determine the executable path
                 ifeq ("$(VENV_ENABLED)", "true")
diff --git a/src/mxmake/topics/applications/zope.mk b/src/mxmake/topics/applications/zope.mk
index 3d6cc4d..13f4afc 100644
--- a/src/mxmake/topics/applications/zope.mk
+++ b/src/mxmake/topics/applications/zope.mk
@@ -82,7 +82,7 @@ $(ZOPE_INSTANCE_TARGET): $(COOKIECUTTER_TARGET) $(ZOPE_CONFIGURATION_FILE)
 	@cookiecutter -f --no-input ${ZOPE_COOKIECUTTER_TEMPLATE_OPTIONS} --config-file $(ZOPE_CONFIGURATION_FILE) --output-dir $(ZOPE_BASE_FOLDER) $(ZOPE_TEMPLATE)
 
 .PHONY: zope-instance
-zope-instance: $(ZOPE_INSTANCE_TARGET) $(SOURCES)
+zope-instance: $(ZOPE_INSTANCE_TARGET) $(SOURCES_TARGET)
 
 .PHONY: zope-start
 zope-start: $(ZOPE_RUN_TARGET)
diff --git a/src/mxmake/topics/core/mxenv.mk b/src/mxmake/topics/core/mxenv.mk
index c352fcf..a5691d5 100644
--- a/src/mxmake/topics/core/mxenv.mk
+++ b/src/mxmake/topics/core/mxenv.mk
@@ -75,7 +75,7 @@
 # mxenv
 ##############################################################################
 
-export OS:=$(OS)
+OS?=
 
 # Determine the executable path
 ifeq ("$(VENV_ENABLED)", "true")