MethodHandles.privateLookupIn() is specified to throw IllegalAccessException if lookup class' module doesn't read target's module. However, IllegalAccessException is not thrown if target's module is unnamed.

Example:
=============================
Sources:

// m/module-info.java
module m {
    exports pkg;
}

// m/pkg/Named.java
package pkg;

import java.lang.invoke.MethodHandles;

public class Named {
    public static MethodHandles.Lookup lookup() {
        return MethodHandles.lookup();
    }
}

// classes/Unnamed.java
import pkg.Named;

import java.lang.invoke.MethodHandles;

public class Unnamed {
    public static void main(String[] args) throws IllegalAccessException {
        MethodHandles.Lookup startingLookup = Named.lookup();
        Class<?> targetClass = Unnamed.class;

        System.out.println("Lookup class module: " + startingLookup.lookupClass().getModule().getName());
        System.out.println("Target class module: " + targetClass.getModule().getName());
        System.out.println("Lookup class module reads target class module: " + startingLookup.lookupClass().getModule().canRead(targetClass.getModule()));

        MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(targetClass, startingLookup);

        System.out.println("Resulting lookup: " + lookup);
        System.out.println("Resulting lookup class: " + lookup.lookupClass());
        System.out.println("Resulting lookup modes: " + lookup.lookupModes());
    }
}

=============================
Output:

Lookup class module: m
Target class module: null
Lookup class module reads target class module: false
Resulting lookup: Unnamed
Resulting lookup class: class Unnamed
Resulting lookup modes: 31