diff --git a/mllib/src/main/scala/org/apache/spark/mllib/evaluation/MulticlassMetrics.scala b/mllib/src/main/scala/org/apache/spark/mllib/evaluation/MulticlassMetrics.scala
index 3b7ba7288c0f3..d48068719a851 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/evaluation/MulticlassMetrics.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/evaluation/MulticlassMetrics.scala
@@ -17,14 +17,13 @@
 
 package org.apache.spark.mllib.evaluation
 
-import org.apache.spark.Logging
+import scala.collection.Map
+
 import org.apache.spark.SparkContext._
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.mllib.linalg.{Matrices, Matrix}
 import org.apache.spark.rdd.RDD
 
-import scala.collection.Map
-
 /**
  * ::Experimental::
  * Evaluator for multiclass classification.
@@ -57,12 +56,12 @@ class MulticlassMetrics(predictionAndLabels: RDD[(Double, Double)]) {
    * as in "labels"
    */
   lazy val confusionMatrix: Matrix = {
-    val transposedMatrix = Array.ofDim[Double](labels.size, labels.size)
+    val transposedFlatMatrix = Array.ofDim[Double](labels.size * labels.size)
     for (i <- 0 to labels.size - 1; j <- 0 to labels.size - 1) {
-      transposedMatrix(i)(j) = confusions.getOrElse((labels(i), labels(j)), 0).toDouble
+      transposedFlatMatrix(i * labels.size + j)
+        = confusions.getOrElse((labels(i), labels(j)), 0).toDouble
     }
-    val flatMatrix = transposedMatrix.flatMap(arr => arr)
-    Matrices.dense(transposedMatrix.length, transposedMatrix(0).length, flatMatrix)
+    Matrices.dense(labels.size, labels.size, transposedFlatMatrix)
   }
 
   /**