Using a Team City environment variable to override a project property

I have a C# project property called Version defined as

<Version Condition="$(Version)==''">1.2.3.4<Version>

1.2.3.4 is the default value.

I have a Team City system property, also called Version, set up to override. So in the custom run dialog in Team City, I can specify a value for Version and that value gets used. This works fine.

If I leave the parameter blank in Team City, however, the default value is still overwritten with blank (null?). If I delete the Team City parameter, the default value is used.

Is the condition incorrect? How can I set up the Team City property to be blank, and only override if I enter some value?