diff --git a/azure-client-runtime/src/main/java/com/microsoft/azure/TaskGroupBase.java b/azure-client-runtime/src/main/java/com/microsoft/azure/TaskGroupBase.java
index 5078b8caa6..e53bfa0a01 100644
--- a/azure-client-runtime/src/main/java/com/microsoft/azure/TaskGroupBase.java
+++ b/azure-client-runtime/src/main/java/com/microsoft/azure/TaskGroupBase.java
@@ -21,7 +21,14 @@
  */
 public abstract class TaskGroupBase<T, U extends TaskItem<T>>
     implements TaskGroup<T, U> {
+    /**
+     * Stores the tasks in this group and their dependency information.
+     */
     private DAGraph<U, DAGNode<U>> dag;
+    /**
+     * ServiceCall instance that aggregate a set of ServiceCalls each associated with
+     * the task in this group.
+     */
     private ParallelServiceCall parallelServiceCall;
 
     /**
@@ -87,7 +94,7 @@ public T taskResult(String taskId) {
     private void executeReadyTasksAsync(final ServiceCallback<T> callback) {
         DAGNode<U> nextNode = dag.getNext();
         while (nextNode != null) {
-            ServiceCall serviceCall = nextNode.data().executeAsync(taskCallback(nextNode, callback));
+            ServiceCall<T> serviceCall = nextNode.data().executeAsync(taskCallback(nextNode, callback));
             this.parallelServiceCall.addCall(serviceCall);
             nextNode = dag.getNext();
         }
@@ -165,8 +172,10 @@ public boolean isCancelled() {
          *
          * @param call the call
          */
-        private void addCall(ServiceCall<?> call) {
-            this.serviceCalls.add(call);
+        private void addCall(ServiceCall call) {
+            if (call != null) {
+                this.serviceCalls.add(call);
+            }
         }
     }
 }
diff --git a/azure-client-runtime/src/main/java/com/microsoft/azure/TaskItem.java b/azure-client-runtime/src/main/java/com/microsoft/azure/TaskItem.java
index 8f0a3459a2..1a4bd9a61b 100644
--- a/azure-client-runtime/src/main/java/com/microsoft/azure/TaskItem.java
+++ b/azure-client-runtime/src/main/java/com/microsoft/azure/TaskItem.java
@@ -38,5 +38,5 @@ public interface TaskItem<U> {
      * @param callback callback to call on success or failure
      * @return the handle of the REST call
      */
-    ServiceCall executeAsync(ServiceCallback<U> callback);
+    ServiceCall<U> executeAsync(ServiceCallback<U> callback);
 }