unary minus exception

groovy> a = 1
groovy> b = -a
groovy>
Exception in thread "main" java.lang.VerifyError: (class:
CommandLine1, method: run signature: ()Ljava/lang/Object
Incompatible argument to function
at java.lang.Class.getDeclaredConstructors0(Native
Method)
at
java.lang.Class.privateGetDeclaredConstructors(Class.java:16
10)
at java.lang.Class.getConstructor0(Class.java:1922)
at java.lang.Class.newInstance0(Class.java:278)
at java.lang.Class.newInstance(Class.java:261)
at
org.codehaus.groovy.runtime.InvokerHelper.createScript(Invok
erHelper.java:296)
at
groovy.lang.GroovyShell.evaluate(GroovyShell.java:235)
at
groovy.lang.GroovyShell.evaluate(GroovyShell.java:215)
at
groovy.ui.InteractiveShell.run(InteractiveShell.java:100)
at
groovy.ui.InteractiveShell.main(InteractiveShell.java:71)
at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAcce
ssorImpl.java:39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMe
thodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at
org.codehaus.classworlds.Launcher.launchStandard(Launcher.ja
va:410)
at
org.codehaus.classworlds.Launcher.launch(Launcher.java:344)
at
org.codehaus.classworlds.Launcher.main(Launcher.java:474)