TypeScript: compiler ignores alternative function signature

I'm using lodash with TypeScript 1.8. At one point in the code I want to reduce from an array like this:

export interface IPackage {
    [...]
    dependencies?: IPackageReference[];
    [...]
}

[...]

// pkg implements IPackageReference
lodash.reduce<IPackageReference, boolean>(pkg.dependencies, function(ref: IPackageReference, state: boolean) {
  retu true;
}, true);

However, the compiler exits with a type mismatch:

package.ts(43,39): error TS2345: Argument of type 'IPackageReference[]' is not assignable to parameter of type 'Dictionary<IPackageReference>'.
  Index signature is missing in type 'IPackageReference[]'.

The typings I use however should support Arrays however:

    /**
    * @see _.reduce
    **/
    reduce<T, TResult>(
        collection: Array<T>,
        callback: MemoIterator<T, TResult>,
        thisArg?: any): TResult;

    /**
    * @see _.reduce
    **/
    reduce<T, TResult>(
        collection: List<T>,
        callback: MemoIterator<T, TResult>,
        thisArg?: any): TResult;

    /**
    * @see _.reduce
    **/
    reduce<T, TResult>(
        collection: Dictionary<T>,
        callback: MemoIterator<T, TResult>,
        thisArg?: any): TResult;

How do I get TSC to recognize my parameter as Array?