Companion object cannot access private variable on the class

A rather weird behavior coming from the Scala REPL.

Although the following compiles without a problem:

class CompanionObjectTest {
    private val x = 3
}
object CompanionObjectTest {
    def testMethod(y:CompanionObjectTest) = y.x + 3
}

the private variable does not seem to be accessible from the companion object in REPL:

scala> class CompanionObjectTest {
     | 
     | private val x = 3;
     | }
defined class CompanionObjectTest

scala> object CompanionObjectTest {
     | 
     | def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
<console>:9: error: value x in class CompanionObjectTest cannot be accessed in CompanionObjectTest
       def testMethod(y:CompanionObjectTest) = y.x + 3
                                                 ^

Why is that happening?