What is wrong with the following Caml query

I have the following table with columns:

Editor | _UIVersionString | Title | LookupNumber | Author 
  a    |    b             | Title1|   100;800    | admin

I use the following query caml

<Or>
<Or>
<Contains>
<FieldRef Name="Editor"/>
<Value Type="Text">a</Value>
</Contains>

<Contains>
<FieldRef Name="_UIVersionString"/>
<Value Type="Text">a</Value>
</Contains>

</Or>

<Or>
<Or>
<Contains>
<FieldRef Name="Title"/>
<Value Type="Text">a</Value>
</Contains>
<Contains>
<FieldRef Name="Lookup" />
<Value Type="Lookup">a</Value>
</Contains>
</Or>

<Or>
<Contains>
<FieldRef Name="LookupNumber" />
<Value Type="Text">a</Value>
</Contains>
<Contains><FieldRef Name="Author"/>
<Value Type="Text">a</Value>
</Contains>
</Or>

</Or>
</Or>

But this always throws an error! Why???????