diff --git a/promql/parser/ast.go b/promql/parser/ast.go
index 4e8f4daa5f..d9117b988e 100644
--- a/promql/parser/ast.go
+++ b/promql/parser/ast.go
@@ -382,11 +382,16 @@ func Walk(ctx context.Context, v Visitor, s *EvalStmt, node Node, path []Node, n
 }
 
 func ExtractSelectors(expr Expr) [][]*labels.Matcher {
-	var selectors [][]*labels.Matcher
+	var (
+		selectors [][]*labels.Matcher
+		l         sync.Mutex
+	)
 	Inspect(context.TODO(), &EvalStmt{Expr: expr}, func(node Node, _ []Node) error {
 		vs, ok := node.(*VectorSelector)
 		if ok {
+			l.Lock()
 			selectors = append(selectors, vs.LabelMatchers)
+			l.Unlock()
 		}
 		return nil
 	}, nil)