diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeMessages.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeMessages.java
index 8236d3dcb1..92b4150cd0 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeMessages.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeMessages.java
@@ -23,7 +23,8 @@ public class ChangeMessages extends TranslationBundle {
   }
 
   public String revertChangeDefaultMessage;
-  public String reviewerNotFound;
+  public String reviewerNotFoundUser;
+  public String reviewerNotFoundUserOrGroup;
 
   public String groupIsNotAllowed;
   public String groupHasTooManyMembers;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
index cd62e45b31..ae440c82e1 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
@@ -209,7 +209,7 @@ public class PostReview implements RestModifyView<RevisionResource, ReviewInput>
         reviewerInput.notify = NotifyHandling.NONE;
 
         PostReviewers.Addition result = postReviewers.prepareApplication(
-            revision.getChangeResource(), reviewerInput);
+            revision.getChangeResource(), reviewerInput, true);
         reviewerJsonResults.put(reviewerInput.reviewer, result.result);
         if (result.result.error != null) {
           hasError = true;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
index f0af5da953..0cdddca11c 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
@@ -147,7 +147,7 @@ public class PostReviewers
       throw new BadRequestException("missing reviewer field");
     }
 
-    Addition addition = prepareApplication(rsrc, input);
+    Addition addition = prepareApplication(rsrc, input, true);
     if (addition.op == null) {
       return addition.result;
     }
@@ -161,18 +161,24 @@ public class PostReviewers
     return addition.result;
   }
 
-  public Addition prepareApplication(ChangeResource rsrc, AddReviewerInput input)
-      throws OrmException, RestApiException, IOException {
+  public Addition prepareApplication(ChangeResource rsrc,
+      AddReviewerInput input, boolean allowGroup)
+          throws OrmException, RestApiException, IOException {
     Account.Id accountId;
     try {
       accountId = accounts.parse(input.reviewer).getAccountId();
     } catch (UnprocessableEntityException e) {
-      try {
-        return putGroup(rsrc, input);
-      } catch (UnprocessableEntityException e2) {
-        throw new UnprocessableEntityException(MessageFormat
-            .format(ChangeMessages.get().reviewerNotFound, input.reviewer));
+      if (allowGroup) {
+        try {
+          return putGroup(rsrc, input);
+        } catch (UnprocessableEntityException e2) {
+          throw new UnprocessableEntityException(MessageFormat.format(
+              ChangeMessages.get().reviewerNotFoundUserOrGroup,
+              input.reviewer));
+        }
       }
+      throw new UnprocessableEntityException(MessageFormat
+          .format(ChangeMessages.get().reviewerNotFoundUser, input.reviewer));
     }
     return putAccount(input.reviewer, reviewerFactory.create(rsrc, accountId),
         input.state(), input.notify);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutAssignee.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutAssignee.java
index 50024362ca..42328a4453 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutAssignee.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutAssignee.java
@@ -91,7 +91,7 @@ public class PutAssignee implements
     reviewerInput.state = ReviewerState.CC;
     reviewerInput.confirmed = true;
     reviewerInput.notify = NotifyHandling.NONE;
-    return postReviewers.prepareApplication(rsrc, reviewerInput);
+    return postReviewers.prepareApplication(rsrc, reviewerInput, false);
   }
 
   @Override
diff --git a/gerrit-server/src/main/resources/com/google/gerrit/server/change/ChangeMessages.properties b/gerrit-server/src/main/resources/com/google/gerrit/server/change/ChangeMessages.properties
index f05f23b63c..f34c9928ba 100644
--- a/gerrit-server/src/main/resources/com/google/gerrit/server/change/ChangeMessages.properties
+++ b/gerrit-server/src/main/resources/com/google/gerrit/server/change/ChangeMessages.properties
@@ -1,7 +1,8 @@
 # Changes to this file should also be made in
 # gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.properties
 revertChangeDefaultMessage = Revert \"{0}\"\n\nThis reverts commit {1}.
-reviewerNotFound = {0} does not identify a registered user or group
+reviewerNotFoundUser = {0} does not identify a registered user
+reviewerNotFoundUserOrGroup = {0} does not identify a registered user or group
 
 groupIsNotAllowed =  The group {0} cannot be added as reviewer.
 groupHasTooManyMembers = The group {0} has too many members to add them all as reviewers.