Android Unit Test - ClassdefNotFound

I am trying to write a unit test for Android, but I get a "NoClassdefFound"-error when executing following very simple code in one of my test methods:

public void testAAA(){
    testAPI1 test = new testAPI1();
    test.makeApiCall1();
}

The class testAPI1 looks like this:

  public class testAPI1 implements SomeInterface{
   public void makeApiCall1(){
      //do something
  }

}

SomeInterface is, well, just some interface. When I remove "SomeInterface", however, everything works fine.

The manifest file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.mamlambo.article.simplecalc.test" android:versionCode="1"
android:versionName="1.0">
<application android:icon="@drawable/icon"
    android:label="@string/app_name">
    <uses-library android:name="android.test.runner" />
</application>

<uses-sdk android:minSdkVersion="7" />
<instrumentation android:targetPackage="com.mamlambo.article.simplecalc"
    android:name="android.test.InstrumentationTestRunner" 
    android:label="Simple Calc Test"/>
 </manifest> 

What exactly is the problem?

The issue was that the Interface I implemented was not found. The interface was in another project I referenced, which wasn't properly referenced apparently. What I did is export that project into a jar-file (without the AndroidManifest!), then imported it in my Test-Project, and everything worked fine from then on.