xsd two alternative optional tag

I am trying to validate some xml like this:

     <Rpms>
        <Rpm>path/file.rpm</Rpm>
        <SlImport>path/file.xml</SlImport>
     <Rpms>

The tags Rpm and SlImport follow these rule:

1. are both optional


2. may appear more than once


3. may appear in any order, whether or not mixed


4. at least one (Rpm or SlImport) tag must be present

I wrote this xsd:

  <xs:element name="Rpms">
    <xs:complexType>
      <xs:sequence>
        <xs:choice maxOccurs="unbounded">
          <xs:element name="SlImport" type="SlElement" minOccurs="0" />
          <xs:element name="Rpm" type="rpmElement" maxOccurs="unbounded">
        </xs:choice>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

It works fine, except in this case: it doesn't throw error if Rpms tag is empty.