emberjs/ember.js · error · Exception

Unexpected node type "${value.type}" found when accepting ${

Error message

Unexpected node type "${value.type}" found when accepting ${name} on ${node.type}

What it means

The visitor's acceptKey sanity check (active when the visitor is mutating) verifies that any node produced while traversing a key has a registered visitor handler. If a child value has an unknown node type, it throws rather than silently corrupting the AST during mutation passes.

Source

Thrown at packages/@handlebars/parser/lib/visitor.js:18

import Exception from './exception.js';

function Visitor() {
  this.parents = [];
}

Visitor.prototype = {
  constructor: Visitor,
  mutating: false,

  // Visits a given value. If mutating, will replace the value if necessary.
  acceptKey: function (node, name) {
    let value = this.accept(node[name]);
    if (this.mutating) {
      // Hacky sanity check: This may have a few false positives for type for the helper
      // methods but will generally do the right thing without a lot of overhead.
      if (value && !Visitor.prototype[value.type]) {
        throw new Exception(
          'Unexpected node type "' +
            value.type +
            '" found when accepting ' +
            name +
            ' on ' +
            node.type
        );
      }
      node[name] = value;
    }
  },

  // Performs an accept operation with added sanity check to ensure
  // required keys are not removed.
  acceptRequired: function (node, name) {
    this.acceptKey(node, name);

    if (!node[name]) {

View on GitHub (pinned to 26f97246a8)

Solutions

Fix the transform to only insert nodes with valid, registered types (e.g. 'MustacheStatement', 'PathExpression')
Check the type string for typos on hand-constructed nodes
Update visitor/transform packages so versions match the parser AST spec
Temporarily run non-mutating to inspect the AST and find the offending node

Example fix

// before
node.params.push({ type: 'mustache', original: 'x' });
// after
node.params.push({ type: 'SubExpression', path: { type: 'PathExpression', original: 'x', data: false, depth: 0, parts: ['x'], loc: null }, params: [], hash: { type: 'Hash', pairs: [], loc: null }, loc: null });

Defensive patterns

Strategy: validation

Validate before calling

const VALID = new Set(['Program','MustacheStatement','BlockStatement','PartialBlockStatement','PartialStatement','CommentStatement','MustacheCommentStatement','DecoratorBlock','Decorator','ElementNode','AttrNode','TextNode','ConcatStatement','SubExpression','PathExpression','Hash','HashPair','BooleanLiteral','NumberLiteral','StringLiteral','UndefinedLiteral','NullLiteral','Block']);
function validNode(n) { return n == null || VALID.has(n.type); }

Type guard

function isAstNode(v) { return v != null && typeof v.type === 'string' && Visitor.prototype[v.type] !== undefined; }

Try / catch

try { visitor.visit(ast); } catch (e) { if (/Unexpected node type/.test(e.message)) { dumpAstAtError(ast, e.message); } throw e; }

Prevention

Only insert nodes whose type exists in the visitor map
Copy node shapes from the parser's own output, don't hand-roll
Keep transform/parser package versions in sync
Run non-mutating traversal in tests to validate ASTs

When it happens

Trigger: A mutating Visitor pass (whitespace control, transforms, ember-template-recast-style edits) sets a node key to a value whose .type has no Visitor.prototype handler, or hand-builds AST nodes with a typo'd type field.

Common situations: Custom AST transforms inserting malformed nodes; plugins/glint/codemods operating on templates; version mismatch where a new node type isn't handled by an older visitor.

Related errors

AI-assisted analysis of emberjs/ember.js@26f97246a8 (2026-09-01). Data as JSON: /api/errors/b1aa54d392cac8a0. Report an issue: GitHub.