Flex ant task problem

When I try to build a project that uses Flex and where the Flex SDK is on one drive and the project files on another I get the following error in ant:

[mxmlc] java.lang.NoClassDefFoundError: flex2/tools/Compiler
[mxmlc] Exception in thread "main"

It seems to be a problem with the Flex mxmlc ant task. Anyone know the root cause of this? It works if I move the Flex SDK over to the project folder but I don't like being hackish like that…

Ant in verbose mode gives:

compile.main:
dropping E:\c from path as it doesn't exist
dropping E:\programs\flex-sdk\lib\mxmlc.jar from path as it doesn't exist
    [mxmlc] java.lang.NoClassDefFoundError: flex2/tools/Compiler
    [mxmlc] Exception in thread "main" 
  [antcall] Exiting E:\svn\trunk\dev\modules\web\webapps\lpjserver\lpjserver-build.xml.

BUILD FAILED
E:\svn\trunk\dev\modules\web\webapps\lpjserver\lpjserver-build.xml:379: The following error occurred while executing this line:
E:\svn\trunk\dev\modules\web\webapps\lpjserver\lpjserver-build.xml:199: mxmlc task failed.
    at org.apache.tools.ant.ProjectHelper.addLocationToBuildException(ProjectHelper.java:508)
    at org.apache.tools.ant.taskdefs.Ant.execute(Ant.java:418)
    at org.apache.tools.ant.taskdefs.CallTarget.execute(CallTarget.java:105)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
    at org.apache.tools.ant.Task.perform(Task.java:348)
    at org.apache.tools.ant.Target.execute(Target.java:357)
    at org.apache.tools.ant.Target.performTasks(Target.java:385)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1337)
    at org.apache.tools.ant.Project.executeTarget(Project.java:1306)
    at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1189)
    at org.apache.tools.ant.Main.runBuild(Main.java:758)
    at org.apache.tools.ant.Main.startAnt(Main.java:217)
    at org.apache.tools.ant.launch.Launcher.run(Launcher.java:257)
    at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)
Caused by: E:\svn\trunk\dev\modules\web\webapps\lpjserver\lpjserver-build.xml:199: mxmlc task failed.
    at flex.ant.FlexTask.executeOutOfProcess(FlexTask.java:253)
    at flex.ant.FlexTask.execute(FlexTask.java:223)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
    at org.apache.tools.ant.Task.perform(Task.java:348)
    at org.apache.tools.ant.Target.execute(Target.java:357)
    at org.apache.tools.ant.Target.performTasks(Target.java:385)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1337)
    at org.apache.tools.ant.helper.SingleCheckExecutor.executeTargets(SingleCheckExecutor.java:38)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1189)
    at org.apache.tools.ant.taskdefs.Ant.execute(Ant.java:416)
    ... 17 more
--- Nested Exception ---
E:\svn\trunk\dev\modules\web\webapps\lpjserver\lpjserver-build.xml:199: mxmlc task failed.
    at flex.ant.FlexTask.executeOutOfProcess(FlexTask.java:253)
    at flex.ant.FlexTask.execute(FlexTask.java:223)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
    at org.apache.tools.ant.Task.perform(Task.java:348)
    at org.apache.tools.ant.Target.execute(Target.java:357)
    at org.apache.tools.ant.Target.performTasks(Target.java:385)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1337)
    at org.apache.tools.ant.helper.SingleCheckExecutor.executeTargets(SingleCheckExecutor.java:38)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1189)
    at org.apache.tools.ant.taskdefs.Ant.execute(Ant.java:416)
    at org.apache.tools.ant.taskdefs.CallTarget.execute(CallTarget.java:105)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
    at org.apache.tools.ant.Task.perform(Task.java:348)
    at org.apache.tools.ant.Target.execute(Target.java:357)
    at org.apache.tools.ant.Target.performTasks(Target.java:385)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1337)
    at org.apache.tools.ant.Project.executeTarget(Project.java:1306)
    at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1189)
    at org.apache.tools.ant.Main.runBuild(Main.java:758)
    at org.apache.tools.ant.Main.startAnt(Main.java:217)
    at org.apache.tools.ant.launch.Launcher.run(Launcher.java:257)
    at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)