diff --git a/test/treegen.lua b/test/treegen.lua
index 987ff20df4d9c637348f2381b10e900bee49c5dc..5d41065f56de49f931d0373872646244819cf6a0 100644
--- a/test/treegen.lua
+++ b/test/treegen.lua
@@ -79,7 +79,7 @@ end
 -- unless KEEP_DATA environment variable is set to a
 -- non-empty value.
 function treegen.clean(g)
-    local dirs = table.copy(g.tempdirs)
+    local dirs = table.copy(g.tempdirs) or {}
     g.tempdirs = nil
 
     local keep_data = (os.getenv('KEEP_DATA') or '') ~= ''