How to call some Ant target only if some environment variable was not set?

Question

I would like do not call some target in build.xml only in case if there is some variable on enrironment.
Following code does not work:

<property environment="env"/>
<property name="app.mode" value="${env.APP_MODE}"/>

<target name="someTarget" unless="${app.mode}">    
   ...
</target>

<target name="all" description="Creates app">
   <antcall target="someTarget" />
</target>

Target "someTarget" executes regardless of whether there is environment variable APP_MODE or not.

PS.
We use Ant 1.7.0.

UPDATE:
Question was corrected.

ANSWER:
Use unless="app.mode", but not unless="${app.mode}".
http://ant.apache.org/manual/properties.html#if+unless
In Ant 1.7.1 and earlier, these attributes could only be property names.
As of Ant 1.8.0, you may instead use property expansion; a value of true (or on or yes) will enable the item, while false (or off or no) will disable it.
Other values are still assumed to be property names and so the item is enabled only if the named property is defined.

Answer 1

The docs for the unlessattribute say:

the name of the property that must not be set in order for this target to execute, or something evaluating to false

So in your case, you need to put the name of the property, rather than an evaluation of the property:

<target name="someTarget" unless="app.mode">    
   ...
</target>