Trying to pass variable from TFS to WiX

gistfile1.txt

I have a Variable defined in my TFS Build template
Name:TFSGenProductVersion 
Type:String 
Scope:Sequence 
Default:"3.1.2"

I added this to the wixproj

<PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
  <DefineConstants>Debug;MyProductVersion=0.0.0.1</DefineConstants>
</PropertyGroup>
<PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
  <DefineConstants>MyProductVersion=$(TFSGenProductVersion)</DefineConstants>
</PropertyGroup>

I added this to the wxs

<Product Id="B5F53C50-A6D9-4731-BE9B-AA3B7CF63366" 
           Name="MyApp" 
           Language="1033"
           Version="$(var.MyProductVersion)" 
           Manufacturer="Me" 
           UpgradeCode="B7CE3354-C478-4E0B-8547-A83AC7F3F77C">
           
when TFS builds I get the following, so I know the variable is not being passed in.

 ..\Product.wxs (12): The Product/@Version attribute's value cannot be an empty string.  If you want the value to be null or empty, simply remove the entire attribute.
 ..\Product.wxs (12): The Product/@Version attribute was not found; it is required.