xml schema restriction list with different sub elements by value

I like to create an element that has different subelements depending on the value of one of these elements. I wonder if and how I can achieve this (best possible solution). So if the Type is piece there must be a Weight element. If the Type is kilogram or gram or ... it does not need it.

    <?xml version="1.0" encoding="UTF-8"?>
    <Amounts>
        <Amount>
            <Type>piece</Type>
            <Value>6</Value>
            <Weight>
                <Value>1.5</Value>
                <Type>liter</Type>
            </Weight>
        </Amount>
        <Amount>
            <Type>kilogram</Type>
            <Value>0.610</Value>
        </Amount>
    </Amounts>

So I thought I might do something like this, but I get errors (see below).

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://ift.tt/tphNwY" targetNamespace="http://ift.tt/1vQ9BYc"
    xmlns:amount="http://ift.tt/1vQ9BYc" elementFormDefault='qualified'>
    <element name="Amount">
        <complexType>
            <choice>
                <group ref="amount:kilogram" />
                <group ref="amount:piece" />
            </choice>
        </complexType>
    </element>
    <group name="kilogram">
        <sequence>
            <element name="Type" fixed="kilogram" />
            <element name="Value" type="float" />
        </sequence>
    </group>
    <group name="piece">
        <sequence>
            <element name="Type" fixed="piece" />
            <element name="Value" type="int" />
            <element name="Weight">
                <complexType>
                    <sequence>
                        <element name="Type" type="amount:amountType" />
                        <element name="Value" type="float" />
                    </sequence>
                </complexType>
            </element>
        </sequence>
    </group>
    <simpleType name="amountType">
        <restriction base="string">
            <enumeration value="kg" />
            <enumeration value="g" />
            <enumeration value="mg" />
            <enumeration value="lb" />
        </restriction>
    </simpleType>
</schema>

Error I get at line 5 is:

Multiple annotations found at this line: - cos-nonambig: "http://ift.tt/1vQ9BYc":Type and "http://ift.tt/1vQ9BYc":Type (or elements from their substitution group) violate "Unique Particle Attribution". During validation

against this schema, ambiguity would be created for those two particles. - cos-element-consistent: Error for type '#AnonType_Amount'. Multiple elements with name 'Value', with different types, appear in the model group.