Invalid block parameters syntax: empty parameters list, expe

Error message

Invalid block parameters syntax: empty parameters list, expecting at least one identifier

What it means

An empty block parameters list (`<Foo as ||>` or `<Foo as | |>`) is rejected. Following Handlebars, at least one identifier must appear between the pipes, so the parser throws when the closing pipe is reached with no params collected.

Source

Thrown at packages/@glimmer/syntax/lib/parser/tokenizer-event-handlers.ts:421

          state = { state: 'Done' };
          this.pendingError = {
            mustache(loc: src.SourceSpan) {
              throw generateSyntaxError(
                `Invalid block parameters syntax: mustaches cannot be used inside parameters list`,
                loc
              );
            },
            eof(loc: src.SourceOffset) {
              throw generateSyntaxError(
                `Invalid block parameters syntax: expecting the tag to be closed with ">" or "/>" after parameters list`,
                as.start.until(loc)
              );
            },
          };
        } else if (next === '|') {
          if (element.params.length === 0) {
            // Following Handlebars and treat empty block params a syntax error
            throw generateSyntaxError(
              `Invalid block parameters syntax: empty parameters list, expecting at least one identifier`,
              as.start.until(this.offset().move(1))
            );
          } else {
            state = { state: 'AfterEndPipe' };
            this.tokenizer.consume();
          }
        } else if (next === '>' || next === '/') {
          throw generateSyntaxError(
            `Invalid block parameters syntax: incomplete parameters list, expecting "|" but the tag was closed prematurely`,
            as.start.until(this.offset().move(1))
          );
        } else {
          // slurp up anything else into the name, validate later
          state = {
            state: 'BlockParamName',
            name: next,
            start: this.offset(),

View on GitHub (pinned to 26f97246a8)

Solutions

Add at least one identifier between the pipes, e.g. `<Foo as |a|>`
Remove the `as | |` entirely if no block params are needed: `<Foo>`

Example fix

// before
<Foo as ||>
// after
<Foo as |item|>

Defensive patterns

Strategy: validation

Validate before calling

// Reject empty block param lists
if (/<[^>]*\bas\s\|\s*\|/.test(template)) {
  throw new Error('Empty block params list; expected at least one identifier');
}

Prevention

Remove `as | |` entirely when no params are needed
Code-review templates for placeholder empty params

When it happens

Trigger: parsePossibleBlockParams (via appendToAttributeName) when the next character is `|` and element.params.length === 0 — templates like `<Foo as ||>`.

Common situations: Writing `as ||` as a placeholder intending to add params later; mechanically deleting a param but leaving both pipes.

Related errors

AI-assisted analysis of emberjs/ember.js@26f97246a8 (2026-09-01). Data as JSON: /api/errors/acce77b43f323497. Report an issue: GitHub.