Ant and Jacoco: Need to set junit's fork to "true" but not property that's true

I'm using the Jacoco Junit task and tried the following:

    <!-- run the junit tests -->
    <echo>DEBUG: $${junit.fork} = "${junit.fork}"</echo>
    <jacoco:coverage
        destfile="${target.dir}/jacoco.exec"
        append="false">
        <junit fork="${junit.fork}" includeAntRuntime="true">
            <classpath>
                <pathelement path="${main.destdir}"/>
                <pathelement path="${test.destdir}"/>
            </classpath>
            <classpath refid="test.classpath"/>
            <formatter type="${junit.formatter.type}"/>
            <batchtest      todir="${junit.batchtest.todir}">
                <fileset dir="${test.destdir}" />
            </batchtest>
        </junit>
    </jacoco:coverage>

And got the following:

test:
     [echo] DEBUG: ${junit.fork} = "true"
[jacoco:coverage] Enhancing junit with coverage

BUILD FAILED
D:\build.xml:233: Coverage can only be applied on a forked VM

Total time: 6 seconds

D:\>

As you can see, ${junit.fork} property was set to true, and I used that property in <junit fork="${junit.fork}"/>.

However, instead of using that property, I simply set <junit fork="true">, it works fine:

    <jacoco:coverage
        destfile="${target.dir}/jacoco.exec"
        append="false">
        <junit fork="true" includeAntRuntime="true">
            <classpath>
                <pathelement path="${main.destdir}"/>
                <pathelement path="${test.destdir}"/>
            </classpath>
            <classpath refid="test.classpath"/>
            <formatter type="${junit.formatter.type}"/>
            <batchtest      todir="${junit.batchtest.todir}">
                <fileset dir="${test.destdir}" />
            </batchtest>
        </junit>
    </jacoco:coverage>

I have run ant -d test to verify that the Java JVM is forking when I use the ${junit.fork} property.

Why is JaCoCo insisting that the JUnit test isn't being forked unless I set the fork parameter to the string true and not to a property that's equals true?