diff --git a/promql/ast.go b/promql/ast.go
index af3ed4189..0c8a3ad2a 100644
--- a/promql/ast.go
+++ b/promql/ast.go
@@ -237,57 +237,80 @@ type VectorMatching struct {
 
 // Visitor allows visiting a Node and its child nodes. The Visit method is
 // invoked for each node with the path leading to the node provided additionally.
-// If the result visitor w is not nil, Walk visits each of the children
+// If the result visitor w is not nil and no error, Walk visits each of the children
 // of node with the visitor w, followed by a call of w.Visit(nil, nil).
 type Visitor interface {
-	Visit(node Node, path []Node) (w Visitor)
+	Visit(node Node, path []Node) (w Visitor, err error)
 }
 
 // Walk traverses an AST in depth-first order: It starts by calling
 // v.Visit(node, path); node must not be nil. If the visitor w returned by
-// v.Visit(node, path) is not nil, Walk is invoked recursively with visitor
-// w for each of the non-nil children of node, followed by a call of
-// w.Visit(nil).
+// v.Visit(node, path) is not nil and the visitor returns no error, Walk is
+// invoked recursively with visitor w for each of the non-nil children of node,
+// followed by a call of w.Visit(nil), returning an error
 // As the tree is descended the path of previous nodes is provided.
-func Walk(v Visitor, node Node, path []Node) {
-	if v = v.Visit(node, path); v == nil {
-		return
+func Walk(v Visitor, node Node, path []Node) error {
+	var err error
+	if v, err = v.Visit(node, path); v == nil || err != nil {
+		return err
 	}
 	path = append(path, node)
 
 	switch n := node.(type) {
 	case Statements:
 		for _, s := range n {
-			Walk(v, s, path)
+			if err := Walk(v, s, path); err != nil {
+				return err
+			}
 		}
 	case *AlertStmt:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case *EvalStmt:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case *RecordStmt:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case Expressions:
 		for _, e := range n {
-			Walk(v, e, path)
+			if err := Walk(v, e, path); err != nil {
+				return err
+			}
 		}
 	case *AggregateExpr:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case *BinaryExpr:
-		Walk(v, n.LHS, path)
-		Walk(v, n.RHS, path)
+		if err := Walk(v, n.LHS, path); err != nil {
+			return err
+		}
+		if err := Walk(v, n.RHS, path); err != nil {
+			return err
+		}
 
 	case *Call:
-		Walk(v, n.Args, path)
+		if err := Walk(v, n.Args, path); err != nil {
+			return err
+		}
 
 	case *ParenExpr:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case *UnaryExpr:
-		Walk(v, n.Expr, path)
+		if err := Walk(v, n.Expr, path); err != nil {
+			return err
+		}
 
 	case *MatrixSelector, *NumberLiteral, *StringLiteral, *VectorSelector:
 		// nothing to do
@@ -296,21 +319,23 @@ func Walk(v Visitor, node Node, path []Node) {
 		panic(fmt.Errorf("promql.Walk: unhandled node type %T", node))
 	}
 
-	v.Visit(nil, nil)
+	_, err = v.Visit(nil, nil)
+	return err
 }
 
-type inspector func(Node, []Node) bool
+type inspector func(Node, []Node) error
 
-func (f inspector) Visit(node Node, path []Node) Visitor {
-	if f(node, path) {
-		return f
+func (f inspector) Visit(node Node, path []Node) (Visitor, error) {
+	if err := f(node, path); err == nil {
+		return f, nil
+	} else {
+		return nil, err
 	}
-	return nil
 }
 
 // Inspect traverses an AST in depth-first order: It starts by calling
-// f(node, path); node must not be nil. If f returns true, Inspect invokes f
+// f(node, path); node must not be nil. If f returns a nil error, Inspect invokes f
 // for all the non-nil children of node, recursively.
-func Inspect(node Node, f func(Node, []Node) bool) {
+func Inspect(node Node, f inspector) {
 	Walk(inspector(f), node, nil)
 }
diff --git a/promql/engine.go b/promql/engine.go
index 2206607c4..adf3bf00c 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -451,7 +451,7 @@ func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *EvalStmt) (
 
 func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *EvalStmt) (storage.Querier, error) {
 	var maxOffset time.Duration
-	Inspect(s.Expr, func(node Node, _ []Node) bool {
+	Inspect(s.Expr, func(node Node, _ []Node) error {
 		switch n := node.(type) {
 		case *VectorSelector:
 			if maxOffset < LookbackDelta {
@@ -468,7 +468,7 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *Ev
 				maxOffset = n.Offset + n.Range
 			}
 		}
-		return true
+		return nil
 	})
 
 	mint := s.Start.Add(-maxOffset)
@@ -478,7 +478,7 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *Ev
 		return nil, err
 	}
 
-	Inspect(s.Expr, func(node Node, path []Node) bool {
+	Inspect(s.Expr, func(node Node, path []Node) error {
 		var set storage.SeriesSet
 		params := &storage.SelectParams{
 			Step: int64(s.Interval / time.Millisecond),
@@ -491,13 +491,13 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *Ev
 			set, err = querier.Select(params, n.LabelMatchers...)
 			if err != nil {
 				level.Error(ng.logger).Log("msg", "error selecting series set", "err", err)
-				return false
+				return err
 			}
 			n.series, err = expandSeriesSet(set)
 			if err != nil {
 				// TODO(fabxc): use multi-error.
 				level.Error(ng.logger).Log("msg", "error expanding series set", "err", err)
-				return false
+				return err
 			}
 
 		case *MatrixSelector:
@@ -506,15 +506,15 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *Ev
 			set, err = querier.Select(params, n.LabelMatchers...)
 			if err != nil {
 				level.Error(ng.logger).Log("msg", "error selecting series set", "err", err)
-				return false
+				return err
 			}
 			n.series, err = expandSeriesSet(set)
 			if err != nil {
 				level.Error(ng.logger).Log("msg", "error expanding series set", "err", err)
-				return false
+				return err
 			}
 		}
-		return true
+		return nil
 	})
 	return querier, err
 }