reflex-dev/reflex · error · ValueError

For conditional vars, the second argument must be set.

Error message

For conditional vars, the second argument must be set.

What it means

When `rx.cond(condition, c1, c2)` is used with non-component return values (i.e. building a conditional Var rather than a Cond component), the second argument c2 is mandatory because a ternary Var needs both branches. Only component rendering allows an unset c2.

Source

Thrown at packages/reflex-components-core/src/reflex_components_core/core/cond.py:204

    # Convert the condition to a Var.
    cond_var = LiteralVar.create(condition)
    if cond_var is None:
        msg = "The condition must be set."
        raise ValueError(msg)

    # If the first component is a component, create a Cond component.
    if isinstance(c1, BaseComponent):
        if not isinstance(c2, types.Unset) and not isinstance(c2, BaseComponent):
            return Cond.create(cond_var.bool(), c1, Fragment.create(c2))
        return Cond.create(cond_var.bool(), c1, c2)

    # Otherwise, create a conditional Var.
    # Check that the second argument is valid.
    if isinstance(c2, BaseComponent):
        return Cond.create(cond_var.bool(), Fragment.create(c1), c2)
    if isinstance(c2, types.Unset):
        msg = "For conditional vars, the second argument must be set."
        raise ValueError(msg)

    # convert the truth and false cond parts into vars so the _var_data can be obtained.
    c1_var = Var.create(c1)
    c2_var = Var.create(c2)

    if c1_var is cond_var or c1_var.equals(cond_var):
        c1_var = c1_var.to(types.value_inside_optional(c1_var._var_type))

    # Create the conditional var.
    return ternary_operation(
        cond_var.bool(),
        c1_var,
        c2_var,
    )


@overload
def color_mode_cond(light: Component, dark: Component | None = None) -> Component: ...  # pyright: ignore [reportOverlappingOverload]

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Supply the second branch: rx.cond(State.flag, 'yes', 'no')
2. Use rx.cond(State.flag, rx.text('yes')) if you truly want component-only rendering (unset else renders nothing)
3. Use an empty-string default explicitly: rx.cond(flag, 'yes', '')

Example fix

// before
rx.text(rx.cond(State.is_pro, 'Pro'))
// after
rx.text(rx.cond(State.is_pro, 'Pro', 'Free'))

val = rx.cond(flag, 'yes', 'no' if not isinstance(c2_given, str) else c2_given)
# simply: always pass the third argument for value-returning conds

When it happens

Trigger: `rx.cond(State.flag, 'yes')` with no third argument, where c1 is a string/Var rather than a component.

Common situations: Using rx.cond to produce dynamic text or values and forgetting the else branch; assuming the else defaults to empty string.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• The condition must be set.
• Missing 'tag' keyword-argument for Icon
• IconButton requires a child icon. Pass a string as the first
• For `{template_name}` template, `template_code_dir_name` and
• ChildrenTypeError(component=cls.__name__, child=child)

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/2d52701c66901e10. Report an issue: GitHub.