PsiClass.superclass returns null in LightCodeInsightFixtureTestCase

I'm trying to understand why the assertion in the following test fails:

class TmpTest : LightCodeInsightFixtureTestCase() {

    fun test() {
        myFixture.configureByFiles("Child.java", "Parent.java")

        val childClass = myFixture.findClass("test.Child")
        assertThat(childClass.superClass).isNotNull()
    }

    override fun getTestDataPath(): String {
        return "src/test/resources"
    }
}

The child file looks like this:

package test;

class Child extends Parent {}