diff --git a/slam/LinearApproxFactor-inl.h b/slam/LinearApproxFactor-inl.h
index c58ba7950..9518d072f 100644
--- a/slam/LinearApproxFactor-inl.h
+++ b/slam/LinearApproxFactor-inl.h
@@ -42,14 +42,16 @@ LinearApproxFactor<Values,Key>::LinearApproxFactor(
 template <class Values, class Key>
 Vector LinearApproxFactor<Values,Key>::unwhitenedError(const Values& c) const {
 	// extract the points in the new config
-	VectorValues delta;
-	//	BOOST_FOREACH(const Key& key, nonlinearKeys_) {
-	//		X newPt = c[key], linPt = lin_points_[key];
-	//		Vector d = linPt.logmap(newPt);
-	//		delta.insert(key, d);
-	//	}
+	Vector ret = - b_;
 
-	return zero(b_.size()); //FIXME: PLACEHOLDER!
+	BOOST_FOREACH(const Key& key, nonlinearKeys_) {
+		X newPt = c[key], linPt = lin_points_[key];
+		Vector d = linPt.logmap(newPt);
+		const Matrix& A = matrices_.at(Symbol(key));
+		ret += prod(A, d);
+	}
+
+	return ret;
 }
 
 /* ************************************************************************* */
diff --git a/tests/testLinearApproxFactor.cpp b/tests/testLinearApproxFactor.cpp
index f04d6f67d..099be4e44 100644
--- a/tests/testLinearApproxFactor.cpp
+++ b/tests/testLinearApproxFactor.cpp
@@ -1,4 +1,4 @@
-/*
+/**
  * @file testLinearApproxFactor.cpp
  * @brief tests for dummy factor that contains a linear factor
  * @author Alex Cunningham
@@ -26,17 +26,23 @@ TEST ( LinearApproxFactor, basic ) {
 	Ordering ordering;
 	ordering.push_back(key1);
 	planarSLAM::Values lin_points;
+	planarSLAM::PointKey PKey(1);
+	Point2 point(1.0, 2.0);
+	lin_points.insert(PKey, point);
 	ApproxFactor f1(lin_factor, ordering, lin_points);
 
 	EXPECT(f1.size() == 1);
-	ApproxFactor::KeyVector expKeyVec;
-	expKeyVec.push_back(planarSLAM::PointKey(key1.index()));
+	EXPECT(assert_equal(key1, f1.keys().front()));
+	EXPECT(assert_equal(b, f1.get_b()));
 
-	planarSLAM::Values config; // doesn't really need to have any data
+	planarSLAM::Values config;
+	config.insert(PKey, Point2(2.0, 3.0));
 	GaussianFactor::shared_ptr actual = f1.linearize(config, ordering);
 
+	EXPECT(assert_equal(Vector_(2, -0.2, -0.2), f1.unwhitenedError(config)));
+
 	// Check the linearization
-	CHECK(assert_equal(*lin_factor, *actual));
+	EXPECT(assert_equal(*lin_factor, *actual));
 }
 
 /* ************************************************************************* */