diff --git a/promql/engine.go b/promql/engine.go
index b8a8ea095..08c787d5c 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -573,7 +573,8 @@ func (ng *Engine) validateOpts(expr parser.Expr) error {
 	return validationErr
 }
 
-func (ng *Engine) newTestQuery(f func(context.Context) error) Query {
+// NewTestQuery: inject special behaviour into Query for testing.
+func (ng *Engine) NewTestQuery(f func(context.Context) error) Query {
 	qry := &query{
 		q:           "test statement",
 		stmt:        parser.TestStmt(f),