diff --git a/make/py-mod.mk b/make/py-mod.mk
index cf8ae627..54ac1bf6 100644
--- a/make/py-mod.mk
+++ b/make/py-mod.mk
@@ -2,6 +2,7 @@ include $(JWBDIR)/make/defs.mk
 include $(JWBDIR)/make/py-defs.mk
 include $(JWBDIR)/make/dirs.mk
 include $(JWBDIR)/make/dev-utils.mk
+include $(JWBDIR)/make/test-guard.mk
 
 PY_INIT_TMPL                 = $(wildcard __init__.py.tmpl)
 PY_SED_EXTRACT_EXPORT_DEF   := /\(\(class\|def\)\s\+[a-zA-Z_].*\|^ *\S\+\s*=.*\|^\s\+\"\S\+\",\)\s*\#\s*export/!d;
@@ -24,7 +25,7 @@ endif
 
 all: $(PY_PYC)
 install: install-dirs.done install-reg.done
-clean: py.clean
+clean: invalidate-testability py.clean
 distclean:
 
 include $(JWBDIR)/make/py-rules.mk
diff --git a/make/py-run.mk b/make/py-run.mk
index 85ebeba1..c2c8f80f 100644
--- a/make/py-run.mk
+++ b/make/py-run.mk
@@ -1,5 +1,6 @@
 include $(JWBDIR)/make/defs.mk
 include $(JWBDIR)/make/py-defs.mk
+include $(JWBDIR)/make/test-guard.mk
 
 EXE                         ?= $(firstword $(wildcard main.py runme.py test.py *.py))
 EXE_ARGS                    ?=
@@ -9,7 +10,7 @@ all:
 install:
 clean:
 distclean:
-test: all
+test: all testability
 
 run:
 	$(PYTHON_RUNNER) $(EXE) $(EXE_ARGS)
diff --git a/make/test-guard.mk b/make/test-guard.mk
new file mode 100644
index 00000000..db91a623
--- /dev/null
+++ b/make/test-guard.mk
@@ -0,0 +1,21 @@
+# Testability guard
+#
+# Tree testability is a single top-level property tracked by
+# $(TOPDIR)/dirs-all.done: created by a full `make all` from the topdir,
+# removed by `make clean`. A per-dir `make test` only makes sense when the tree
+# is importable, i.e. testable. So `test` requires testability, and a `make
+# clean` that drops artifacts crucial for testing (a Python module's
+# __init__.py, ...) invalidates it.
+
+.PHONY: testability invalidate-testability
+
+TEST_GUARD                   = $(TOPDIR)/dirs-all.done
+
+all:
+
+testability:
+	@test -f "$(TEST_GUARD)" || \
+		{ echo 'Error: Tree not testable -- run "make all" (or "make test") from project root to fix that!' >&2; exit 1; }
+
+invalidate-testability:
+	$(RM) -f "$(TEST_GUARD)"
diff --git a/make/test-jw-pkg.mk b/make/test-jw-pkg.mk
index 32f1d651..d9718863 100644
--- a/make/test-jw-pkg.mk
+++ b/make/test-jw-pkg.mk
@@ -14,6 +14,8 @@ all:
 include $(JWBDIR)/make/defs.mk
 include $(JWBDIR)/make/py-defs.mk
 include $(JWBDIR)/make/dev-utils.mk
+include $(JWBDIR)/make/test-guard.mk
 
-test-default:
+test: testability
+test-default: testability
 	$(TEST_CMD_LINE)