schema: decimal type that allows empty string

[Blake Minghelli]

I am trying to define a required element of type 'decimal' that should allow up to 12 total digits and up to 2 fractional digits. It also must allow empty strings (i.e. element is present but contains no value).

Here is the xsd snippet for the element:

<xs:element name="HighActionAmount" nillable="true">
  <xs:simpleType>
    <xs:restriction base="xs :mrgreen: ecimal">
      <xs:totalDigits value="12"/>
      <xs:fractionDigits value="2"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>  

So, I want this to be a legal value in an instance document:

<HighActionAmount/>  

But when I validate (using Xerces), I get this error:

Datatype error: In element 'HighActionAmount' : '' is not a decimal.

What do I need to do to make the element allow empty values?