diff --git a/src/ckmeans.js b/src/ckmeans.js
index ba8d539b..a92961b8 100644
--- a/src/ckmeans.js
+++ b/src/ckmeans.js
@@ -106,14 +106,14 @@ function ckmeans(data, nClusters) {
                 var squaredDifference = Math.pow(
                     sorted[sortedIdx] - firstClusterMean, 2);
                 matrix[cluster][sortedIdx] = matrix[cluster][sortedIdx - 1] +
-                    ((sortedIdx - 1) / sortedIdx) * squaredDifference;
+                    (sortedIdx / (sortedIdx + 1)) * squaredDifference;
 
                 // We're computing a running mean by taking the previous
                 // mean value, multiplying it by the number of elements
                 // seen so far, and then dividing it by the number of
                 // elements total.
                 var newSum = sortedIdx * firstClusterMean + sorted[sortedIdx];
-                firstClusterMean = newSum / sortedIdx;
+                firstClusterMean = newSum / (sortedIdx + 1);
 
             } else {
 
diff --git a/test/ckmeans.test.js b/test/ckmeans.test.js
index 701197af..3e3a379e 100644
--- a/test/ckmeans.test.js
+++ b/test/ckmeans.test.js
@@ -21,11 +21,13 @@ test('C k-means', function(t) {
     t.deepEqual(example, [[-1, -1, -1, -1], [2, 2, 2], [4, 5, 6]]);
     t.deepEqual(cK([1, 2, 3], 3), [[1], [2], [3]]);
 
-    // is this right?
-    t.deepEqual(cK([1, 2, 2, 3], 3), [[1, 2], [2], [3]]);
-    t.deepEqual(cK([1, 2, 2, 3, 3], 3), [[1, 2], [2], [3, 3]]);
-    t.deepEqual(cK([1, 2, 3, 2, 3], 3), [[1, 2], [2], [3, 3]]);
-    t.deepEqual(cK([3, 2, 3, 2, 1], 3), [[1, 2], [2], [3, 3]]);
+    t.deepEqual(cK([0, 3, 4], 2), [[0], [3, 4]]),
+    t.deepEqual(cK([-3, 0, 4], 2), [[-3, 0], [4]]),
+
+    t.deepEqual(cK([1, 2, 2, 3], 3), [[1], [2, 2], [3]]);
+    t.deepEqual(cK([1, 2, 2, 3, 3], 3), [[1], [2, 2], [3, 3]]);
+    t.deepEqual(cK([1, 2, 3, 2, 3], 3), [[1], [2, 2], [3, 3]]);
+    t.deepEqual(cK([3, 2, 3, 2, 1], 3), [[1], [2, 2], [3, 3]]);
     t.deepEqual(cK([3, 2, 3, 5, 2, 1], 3), [[1, 2, 2], [3, 3], [5]]);
 
     t.deepEqual(cK([0, 1, 2, 100, 101, 103], 2), [[0, 1, 2], [100, 101, 103]]);