Reference to method is ambiguous if block in "if" with "instanceof"

import groovy.transform.CompileStatic

@CompileStatic
class Test {
  def toArray(Object value) {
    def res
    if (value instanceof List)
      res = (value as List).toArray()
    else if (value instanceof String || value instanceof GString)
      res = value.toString().split(',')
    else
      throw new Exception("Not support!")

    return res
  }
}

println new Test().toArray([1,2,3])
println new Test().toArray('1,2,3')

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
ideaGroovyConsole.groovy: 10: [Static type checking] - Reference to method is ambiguous. Cannot choose between [java.lang.String java.lang.String#toString(), java.lang.String groovy.lang.GString#toString()]
 @ line 10, column 33.
               res = value.toString().split(',')
                                   ^
ideaGroovyConsole.groovy: 10: [Static type checking] - Cannot find matching method java.lang.Object#split(java.lang.String). Please check if the declared type is correct and if the method exists.
 @ line 10, column 19.
               res = value.toString().split(',')
                     ^