diff --git a/tests/config/test.cfg b/tests/config/test.cfg
index 50515e2aea4568dbf3ab4b03d137d656fb26ef95..f961ee4db3d9d04d2316140a6add7adb63bc5fcf 100644
--- a/tests/config/test.cfg
+++ b/tests/config/test.cfg
@@ -19,6 +19,7 @@ modules.unload 'ta_signal_query'
 modules.unload 'ta_update'
 
 -- load test
+assert(type(env.TEST_FILE) == 'string')
 log('processing test file %s', env.TEST_FILE)
 local tests = dofile(env.TEST_FILE)