@JsType doesn't compile for Gecko permutation

I'm testing JsInterop with GWT 2.8.0-SNAPSHOT. It works fine with Chrome in Super Dev Mode but it throws a NPE on Firefox.
Below I pasted the interface and the full stacktrace. Should I have to create an issue?

Thanks,

This is the interface:
@JsType
public interface Sketch {
    void load(String s);
    JSONObject save();

    @PrototypeOfJsType
    static class Prototype implements Sketch {
        @Override
        public void load(String s) {}

        @Override
        public JSONObject save() {
            return null;
        }
    }
}

This is the full stacktrace:
[INFO] 2015-06-10 19:19:39,730 user:na INFO  org.hibernate.validator.engine.resolver.DefaultTraversableResolver - Instantiated an instance of org.hibernate.validator.engine.resolver.JPATraversableResolver.
[INFO]    [ERROR] An internal compiler exception occurred
[INFO] com.google.gwt.dev.jjs.InternalCompilerException: Unexpected error during visit.
[INFO]     at com.google.gwt.dev.jjs.ast.JVisitor.translateException(JVisitor.java:111)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:276)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:265)
[INFO]     at com.google.gwt.dev.jjs.ast.JVisitor.accept(JVisitor.java:118)
[INFO]     at com.google.gwt.dev.jjs.ast.JReturnStatement.traverse(JReturnStatement.java:40)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor$ListContext.traverse(JModVisitor.java:88)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.acceptWithInsertRemove(JModVisitor.java:331)
[INFO]     at com.google.gwt.dev.jjs.ast.JBlock.traverse(JBlock.java:92)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.traverse(JModVisitor.java:361)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:273)
[INFO]     at com.google.gwt.dev.jjs.ast.JVisitor.accept(JVisitor.java:139)
[INFO]     at com.google.gwt.dev.jjs.ast.JVisitor.accept(JVisitor.java:135)
[INFO]     at com.google.gwt.dev.jjs.ast.JMethodBody.traverse(JMethodBody.java:83)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.traverse(JModVisitor.java:361)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:273)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:265)
[INFO]     at com.google.gwt.dev.jjs.ast.JMethod.visitChildren(JMethod.java:677)
[INFO]     at com.google.gwt.dev.jjs.ast.JMethod.traverse(JMethod.java:669)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor$ListContextImmutable.traverse(JModVisitor.java:169)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.acceptWithInsertRemoveImmutable(JModVisitor.java:336)
[INFO]     at com.google.gwt.dev.jjs.ast.JClassType.traverse(JClassType.java:116)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.traverse(JModVisitor.java:361)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:273)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:265)
[INFO]     at com.google.gwt.dev.jjs.ast.JProgram.visitModuleTypes(JProgram.java:1159)
[INFO]     at com.google.gwt.dev.jjs.ast.JProgram.traverse(JProgram.java:1124)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.traverse(JModVisitor.java:361)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:273)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:265)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.execImpl(ImplementClassLiteralsAsFields.java:384)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.exec(ImplementClassLiteralsAsFields.java:331)
[INFO]     at com.google.gwt.dev.jjs.JavaToJavaScriptCompiler.precompile(JavaToJavaScriptCompiler.java:1182)
[INFO]     at com.google.gwt.dev.jjs.JavaToJavaScriptCompiler.precompile(JavaToJavaScriptCompiler.java:252)
[INFO]     at com.google.gwt.dev.Precompile.precompile(Precompile.java:254)
[INFO]     at com.google.gwt.dev.Precompile.precompile(Precompile.java:202)
[INFO]     at com.google.gwt.dev.Precompile.precompile(Precompile.java:143)
[INFO]     at com.google.gwt.dev.Compiler.run(Compiler.java:198)
[INFO]     at com.google.gwt.dev.Compiler.run(Compiler.java:157)
[INFO]     at com.google.gwt.dev.Compiler$1.run(Compiler.java:117)
[INFO]     at com.google.gwt.dev.CompileTaskRunner.doRun(CompileTaskRunner.java:55)
[INFO]     at com.google.gwt.dev.CompileTaskRunner.runWithAppropriateLogger(CompileTaskRunner.java:50)
[INFO]     at com.google.gwt.dev.Compiler.main(Compiler.java:124)
[INFO] Caused by: java.lang.NullPointerException
[INFO]     at com.google.gwt.dev.jjs.ast.JProgram.normalizeJsoType(JProgram.java:1046)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.resolveClassLiteralField(ImplementClassLiteralsAsFields.java:456)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.createDependentClassLiteral(ImplementClassLiteralsAsFields.java:369)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.getSuperclassClassLiteral(ImplementClassLiteralsAsFields.java:361)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.createLiteralCall(ImplementClassLiteralsAsFields.java:436)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.resolveClassLiteralField(ImplementClassLiteralsAsFields.java:464)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.resolveClassLiteral(ImplementClassLiteralsAsFields.java:441)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields.access$200(ImplementClassLiteralsAsFields.java:83)
[INFO]     at com.google.gwt.dev.jjs.impl.ImplementClassLiteralsAsFields$NormalizeVisitor.endVisit(ImplementClassLiteralsAsFields.java:223)
[INFO]     at com.google.gwt.dev.jjs.ast.JClassLiteral.traverse(JClassLiteral.java:81)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.traverse(JModVisitor.java:361)
[INFO]     at com.google.gwt.dev.jjs.ast.JModVisitor.accept(JModVisitor.java:273)
[INFO]     ... 40 more
[INFO]       [ERROR] at Sketch.java(13): Sketch$Prototype.class
[INFO]          com.google.gwt.dev.jjs.ast.JClassLiteral
[INFO]       [ERROR] at Sketch.java(13): return Sketch$Prototype.class
[INFO]          com.google.gwt.dev.jjs.ast.JReturnStatement
[INFO]       [ERROR] at Sketch.java(13): {
[INFO]   return Sketch$Prototype.class;
[INFO] }
[INFO]          com.google.gwt.dev.jjs.ast.JBlock
[INFO]       [ERROR] at Sketch.java(13): {
[INFO]   return Sketch$Prototype.class;
[INFO] }
[INFO]          com.google.gwt.dev.jjs.ast.JMethodBody
[INFO]       [ERROR] at Sketch.java(13): com.skymira.eforms.web.client.application.forms.widget.drawing.Sketch$Prototype.getClass()Ljava/lang/Class;
[INFO]          com.google.gwt.dev.jjs.ast.JMethod
[INFO]       [ERROR] at Sketch.java(13): com.skymira.eforms.web.client.application.forms.widget.drawing.Sketch$Prototype (extends Object implements Sketch)
[INFO]          com.google.gwt.dev.jjs.ast.JClassType
[INFO]       [ERROR] at Unknown(0): <JProgram>
[INFO]          com.google.gwt.dev.jjs.ast.JProgram


--
Pablo

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