diff --git a/mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala b/mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala
index ff02e5dd3c253..56caeac05c0c1 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala
@@ -119,11 +119,25 @@ class RowMatrix @Since("1.0.0") (
     val nt = if (n % 2 == 0) ((n / 2) * (n + 1)) else (n * ((n + 1) / 2))
 
     // Compute the upper triangular part of the gram matrix.
-    val GU = rows.treeAggregate(new BDV[Double](nt))(
-      seqOp = (U, v) => {
+    val GU = rows.treeAggregate(null.asInstanceOf[BDV[Double]])(
+      seqOp = (maybeU, v) => {
+        val U =
+          if (maybeU == null) {
+            new BDV[Double](nt)
+          } else {
+            maybeU
+          }
         BLAS.spr(1.0, v, U.data)
         U
-      }, combOp = (U1, U2) => U1 += U2)
+      }, combOp = (U1, U2) =>
+        if (U1 == null) {
+          U2
+        } else if (U2 == null) {
+          U1
+        } else {
+          U1 += U2
+        }
+    )
 
     RowMatrix.triuToFull(n, GU.data)
   }
@@ -136,8 +150,14 @@ class RowMatrix @Since("1.0.0") (
     // This succeeds when n <= 65535, which is checked above
     val nt = if (n % 2 == 0) ((n / 2) * (n + 1)) else (n * ((n + 1) / 2))
 
-    val MU = rows.treeAggregate(new BDV[Double](nt))(
-      seqOp = (U, v) => {
+    val MU = rows.treeAggregate(null.asInstanceOf[BDV[Double]])(
+      seqOp = (maybeU, v) => {
+        val U =
+          if (maybeU == null) {
+            new BDV[Double](nt)
+          } else {
+            maybeU
+          }
 
         val n = v.size
         val na = Array.ofDim[Double](n)
@@ -150,7 +170,15 @@ class RowMatrix @Since("1.0.0") (
 
         BLAS.spr(1.0, new DenseVector(na), U.data)
         U
-      }, combOp = (U1, U2) => U1 += U2)
+      }, combOp = (U1, U2) =>
+        if (U1 == null) {
+          U2
+        } else if (U2 == null) {
+          U1
+        } else {
+          U1 += U2
+        }
+    )
 
     bc.destroy()