Android ClassLoader: loadClass fails if class implements and interface

I'm using the following code to dynamically load a class from an external package:

Object plugin = null;
    String packageName = "unipd.elia.deltapluginaccelerometersensor";
    String className = "unipd.elia.deltapluginaccelerometersensor.AccelerometerPlugin";
    try {
        PackageManager packageManager = getPackageManager();
        ApplicationInfo appInfo = packageManager.getApplicationInfo(packageName, 0);
        DexFile df = new DexFile(appInfo.sourceDir);
        ClassLoader cl = getClassLoader();
        Class classToInvestigate = df.loadClass(className, cl);
        plugin = classToInvestigate.newInstance();


    } catch (Exception e) {
        System.out.println("EXCEPTION");
    }

The problem is the following: if the class I try to load is a "simple" class everything goes fine, but if it implents an interface the loading fails. Here's an example of a class that fails to load:

    package unipd.elia.deltapluginaccelerometersensor;

public class AccelerometerPlugin implements IDeltaPlugin {

    @Override
    public String getPluginName() {
        return "Accelerometer Sensor Plugin";
    }

    @Override
    public String getPluginID() {
        return "delta.plugin.accelerometersensor";
    }

    @Override
    public String getPluginAuthor() {
        return "Me";
    }
}

And the interface it implements:

public interface IDeltaPlugin {

    String getPluginName();
    String getPluginID();
    String getPluginAuthor();
}

Basically the call to loadClass() returns null if the class implements the interface. Remove the "implements" statement and it loads fine.

What am I doing wrong?