diff --git a/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Newest.java b/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Newest.java
index ebed701e704..f1134bb562c 100644
--- a/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Newest.java
+++ b/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Newest.java
@@ -323,6 +323,13 @@ private void migrateFeedbackQuestion(teammates.storage.sqlentity.FeedbackSession
             oldResponses = ofy().load().type(FeedbackResponse.class)
                     .filter("feedbackQuestionId", oldQuestion.getId()).list();
         }
+        
+        if (oldResponses == null || oldResponses.size() == 0) {
+            log(String.format("No responses found for question %s %s in course %s", oldQuestion.getId(), oldQuestion.getQuestionNumber(), newSession.getCourse().getId()));
+            return;
+        }
+
+
         for (FeedbackResponse oldResponse : oldResponses) {
             Section newGiverSection = sectionNameToSectionMap.get(oldResponse.getGiverSection());
             Section newRecipientSection = sectionNameToSectionMap.get(oldResponse.getRecipientSection());
@@ -340,6 +347,11 @@ private void migrateFeedbackResponse(teammates.storage.sqlentity.FeedbackQuestio
 
         // cascade migrate response comments
         List<FeedbackResponseComment> oldComments = responseIdToCommentsMap.get(oldResponse.getId());
+        if (oldComments == null) {
+            log(String.format("No comments found for response %s in course %s", oldResponse.getId(), oldResponse.getCourseId()));
+            return;
+        }
+
         for (FeedbackResponseComment oldComment : oldComments) {
             migrateFeedbackResponseComment(newResponse, oldComment, newGiverSection, newRecipientSection);
         }
diff --git a/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Oldest.java b/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Oldest.java
index 525d754847e..32c5ec0af4b 100644
--- a/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Oldest.java
+++ b/src/client/java/teammates/client/scripts/sql/DataMigrationForCourseEntitySql50Oldest.java
@@ -324,6 +324,12 @@ private void migrateFeedbackQuestion(teammates.storage.sqlentity.FeedbackSession
                     .filter("feedbackQuestionId", oldQuestion.getId()).list();
         }
         log(String.format("Feedback question %d has %d responses associated with it", oldQuestion.getQuestionNumber(), oldResponses.size()));
+
+        if (oldResponses == null || oldResponses.size() == 0) {
+            log(String.format("No responses found for question %s %s in course %s", oldQuestion.getId(), oldQuestion.getQuestionNumber(), newSession.getCourse().getId()));
+            return;
+        }
+
         for (FeedbackResponse oldResponse : oldResponses) {
             Section newGiverSection = sectionNameToSectionMap.get(oldResponse.getGiverSection());
             Section newRecipientSection = sectionNameToSectionMap.get(oldResponse.getRecipientSection());
@@ -341,6 +347,11 @@ private void migrateFeedbackResponse(teammates.storage.sqlentity.FeedbackQuestio
 
         // cascade migrate response comments
         List<FeedbackResponseComment> oldComments = responseIdToCommentsMap.get(oldResponse.getId());
+        if (oldComments == null) {
+            log(String.format("No comments found for response %s in course %s", oldResponse.getId(), oldResponse.getCourseId()));
+            return;
+        }
+
         for (FeedbackResponseComment oldComment : oldComments) {
             migrateFeedbackResponseComment(newResponse, oldComment, newGiverSection, newRecipientSection);
         }
diff --git a/src/main/java/teammates/storage/sqlentity/User.java b/src/main/java/teammates/storage/sqlentity/User.java
index 7b5b8ed1bc0..7fe8850980f 100644
--- a/src/main/java/teammates/storage/sqlentity/User.java
+++ b/src/main/java/teammates/storage/sqlentity/User.java
@@ -22,9 +22,7 @@
  * Represents a User.
  */
 @Entity
-@Table(name = "Users", uniqueConstraints = {
-        @UniqueConstraint(name = "Unique email and courseId", columnNames = { "email", "courseId" })
-})
+@Table(name = "Users")
 @Inheritance(strategy = InheritanceType.JOINED)
 public abstract class User extends BaseEntity {
     @Id