apple/pkl · error · VmTypeMismatchException.Nothing

type mismatch (Nothing type)

Error message

type mismatch (Nothing type)

What it means

When a value must conform to the `nothing` type, it can never be valid: TypeNode's Nothing implementation's executeLazily unconditionally throws VmTypeMismatchException.Nothing, reported as "type mismatch (Nothing type)". `nothing` is Pkl's bottom type used to mark values/properties that must never be set or that are intentionally absent.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java:393

    }
  }

  /** The `nothing` type. */
  public static final class NothingTypeNode extends TypeNode {
    public NothingTypeNode(SourceSection sourceSection) {
      super(sourceSection);
    }

    @Override
    public TypeNode initWriteSlotNode(int slot) {
      // do nothing
      return this;
    }

    @Override
    protected Object executeLazily(VirtualFrame frame, Object value) {
      CompilerDirectives.transferToInterpreter();
      throw new VmTypeMismatchException.Nothing(sourceSection, value);
    }

    @Override
    public Object executeAndSet(VirtualFrame frame, Object value) {
      executeLazily(frame, value);
      // guaranteed to never run (execute will always throw).
      CompilerDirectives.transferToInterpreter();
      throw PklBugException.unreachableCode();
    }

    @Override
    public FrameSlotKind getFrameSlotKind() {
      return FrameSlotKind.Illegal;
    }

    @Override
    public VmTyped getMirror() {
      return MirrorFactories.nothingTypeFactory.create(null);

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the value assignment: a property typed `nothing` must not be given any value in this module/context.
2. If the property should hold a value, change its declared type from `nothing` to the actual expected type.
3. Check the template/abstract module documentation for whether the property is expected to remain unassigned or be overridden in a specific way.
4. If you intended a nullable placeholder, use `Null`-able types (e.g. `String?`) instead of `nothing`.

Example fix

// before (Pkl)
class Impl extends Template {
  reserved: nothing = "oops"
}
// after
class Impl extends Template {
  // do not define `reserved`; it is typed `nothing`
}

// Pkl: ensure nothing-typed properties stay unset
if (hasProperty(reserved)) {
  error("`reserved` is typed nothing and must not be defined")
}

When it happens

Trigger: Assigning or evaluating any value where the expected type is `nothing` — e.g. filling in a property declared `nothing` in an abstract module, or a template expecting an overriding module to leave the property unset (declared `nothing`) but a concrete value was provided.

Common situations: Extending template/abstract modules where a `x: nothing` property means "must not be defined here" but the subclass defines it; misusing `nothing` as if it were a nullable placeholder type.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Error converting property `%s` in Pkl object of type `%s` to
• The top-level value of a YAML stream must have type `Collect
• type constraint mismatch
• type mismatch
• union type mismatch

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/1e45f171196a57b1. Report an issue: GitHub.