Simple Olap Cube and simple query MDX - mdx

Question

I have a sample star schema made in this way:
author (id, name)
book (id, name)
sample_fact_table (id, authorfk, bookfk, quantity)
where obiouvsly authorfk is a FK to author.id and bookfk is FK to book.id.
Dimensions are: "author", "book". Measure is "quantity".
I made this configuration for the cube, using Pentaho Schema Workbench tool:
<Schema name="MySchema">
<Dimension type="StandardDimension" visible="true" name="Author">
<Hierarchy visible="true" hasAll="true" allMemberName="All Authors" primaryKey="id">
<Table name="author">
</Table>
<Level name="Name" visible="true" table="author" column="id" nameColumn="name" uniqueMembers="false">
</Level>
</Hierarchy>
</Dimension>
<Dimension type="StandardDimension" visible="true" name="Book">
<Hierarchy visible="true" hasAll="true" allMemberName="All Books" primaryKey="id">
<Table name="book">
</Table>
<Level name="Name" visible="true" table="book" column="id" nameColumn="name" uniqueMembers="false">
</Level>
</Hierarchy>
</Dimension>
<Cube name="TestCube" visible="true" cache="true" enabled="true">
<Table name="sample_fact_table">
</Table>
<DimensionUsage source="Author" name="Author" visible="true" foreignKey="authorfk">
</DimensionUsage>
<DimensionUsage source="Book" name="Book" visible="true" foreignKey="bookfk">
</DimensionUsage>
<Measure name="quantity" column="quantity" aggregator="sum" visible="true">
</Measure>
</Cube>
</Schema>
If I try to execute the MDX query:
select
Measures.quantity ON COLUMNS,
NON EMPTY Author.Children ON ROWS
from [TestCube]
I have a good result:
Axis #0:
{}
Axis #1:
{[Measures].[quantity]}
Axis #2:
{[author].[Al]}
{[author].[John]}
{[author].[Jack]}
Row #0: 3
Row #1: 9
Row #2: 1
But if instead of Author I query on Book, like this:
select
Measures.quantity ON COLUMNS,
NON EMPTY Book.Children ON ROWS
from [TestCube]
I get this error:
Mondrian Error:Failed to parse query 'select
Measures.quantity ON COLUMNS,
NON EMPTY Book.Children ON ROWS
from [TestCube]'
Mondrian Error:MDX object 'Book' not found in cube 'TestCube'
What I'm doing wrong?
Author and Book are both Dimensions, both declared in the same way, both referenced into the Cube.
Thank you!