# M0137

This error means that you declared a type or class that explicitly or implicitly references
an outer type parameter.

Erroneous code examples:

```motoko
class C<T>(){
  type U = T; // type U mentions parameter T of class C
};
```

```motoko
class D<T>(){
  class E(x : T) {
    public let y : T = x; // class E mentions parameter T of class D in a field
  };
}
```

To avoid this error, try parameterizing the inner types.

```motoko
class C<T>(){
  type U<T1> = T1;
};
```

```motoko
class D<T>(){
  class E<T1>(x : T1) {
    public let y : T1 = x;
  };
}
```

This is a temporary restriction of Motoko that we hope to remove in future.