关于groovy的变量与常量运算结果的问题?报错

运行环境：eclipse4.4  groovy 2.3.6

代码如下：

class Test{
    public static void main(String[] args){
        def p = 3.4
        def q = 1.2
        print (p-q + 10) * 20
    }
}

结果是12.2

Caught: java.lang.NullPointerException: Cannot invoke method multiply() on null object
java.lang.NullPointerException: Cannot invoke method multiply() on null object
at com.zdj.groovy.Test.main(Test.groovy:7)

这是怎么回事？求解答