reflex-dev/reflex · error · ValueError

Match children count mismatch: expected {num_cases + 1} (cas

Error message

Match children count mismatch: expected {num_cases + 1} (cases + default), got {len(self.children)}

What it means

The Match component internally builds one child per case plus a default. At render time the children count must equal num_cases + 1; a mismatch means the component tree was mutated or constructed inconsistently (e.g. children added/removed after create).

Source

Thrown at packages/reflex-components-core/src/reflex_components_core/core/match.py:287

    def _render(self) -> Tag:
        # Reconstruct match_cases and default from self.children, which may have
        # been updated by the compiler walker to include memoized wrappers.
        # self.children contains: [case_1_return, case_2_return, ..., default]
        # self.match_cases contains the conditions as Vars.
        num_cases = len(self.match_cases)
        children_var = passthrough_children_var(self.children)
        if children_var is not None:
            # Auto-memo passthrough body: index into the placeholder array so
            # branch JSX stays on the page side.
            cases_returns = [Bare.create(children_var[i]) for i in range(num_cases)]
            default_return = Bare.create(children_var[num_cases])
        else:
            if len(self.children) != num_cases + 1:
                msg = (
                    f"Match children count mismatch: expected {num_cases + 1} "
                    f"(cases + default), got {len(self.children)}"
                )
                raise ValueError(msg)

            cases_returns = self.children[:num_cases]
            default_return = self.children[num_cases]

        return MatchTag(
            cond=str(self.cond),
            match_cases=[
                ([str(cond) for cond in conditions], return_value.render())
                for (conditions, _), return_value in zip(
                    self.match_cases,
                    cases_returns,
                    strict=True,
                )
            ],
            default=default_return.render(),
        )

    def render(self) -> dict:

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Don't mutate a Match component's children after creation; rebuild it via rx.match(...) with the full case list
2. Ensure the default value is passed so the default child exists
3. If subclassing Match, keep match_cases and children in sync (one child per case plus default)
4. If unreachable via public API, report as a Reflex framework bug

try:
    rendered = match_comp.render()
except ValueError as e:
    if 'children count mismatch' in str(e):
        match_comp = rx.match(...)  # rebuild from scratch
    else:
        raise

When it happens

Trigger: Manually appending/removing children on a rx.match component instance after creation, or subclassing Match and overriding create in a way that desynchronizes match_cases from children.

Common situations: Framework-level bug or manual manipulation of Match.children; not typically produced by normal declarative usage of rx.match.

Related errors

• For cases with return types as Vars, a default case must be
• The condition must be set
• rx.match should have at least one case.
• rx.match should have tuples of cases and one default case as
• A case tuple should have at least a match case element and a

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/80d77c4783ddf078. Report an issue: GitHub.