is possible to declare a nillable decimal?

DISCLAIMER: I searched the archives but didn't find anything about this
so SIA if it's the nth time it's been asked.

Given the example schema and XML input below, I want an empty
<TradeAmount> element to be considered valid, but of course I do not
understand how to make that happen, since the 'nillable' attribute is
documented to only work on base="xsd:string" types and I'm trying to
apply it to a base="xsd:decimal" type. Any hints to the solution?

TIA,
Phil Fuhlman

Example schema:

	<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

		<xsd:simpleType name="myTypes.TradeAmount">
			<xsd:restriction base="xsd:decimal">
				<xsd:fractionDigits value="2"/>
			</xsd:restriction>
		</xsd:simpleType>

		<xsd:complexType name="myTypes.Transaction">
			<xsd:all>
				<xsd:element name="TradeAmount"
type="myTypes.TradeAmount" minOccurs="0" maxOccurs="1" nillable="true"/>
			</xsd:all>
		</xsd:complexType>

		<xsd:element name="Transaction"
type="myTypes.Transaction"/>

	</xsd:schema>

Example XML document that fails validation because <TradeAmount> element
is empty:

	<Transaction>
		<TradeAmount></TradeAmount>
	</Transaction>