diff --git a/java/com/google/gerrit/server/change/ReviewerModifier.java b/java/com/google/gerrit/server/change/ReviewerModifier.java
index d714215..4b1d06c 100644
--- a/java/com/google/gerrit/server/change/ReviewerModifier.java
+++ b/java/com/google/gerrit/server/change/ReviewerModifier.java
@@ -24,6 +24,7 @@
 import static java.util.Comparator.comparing;
 import static java.util.Objects.requireNonNull;
 
+import autovalue.shaded.com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
@@ -222,7 +223,10 @@
       throws IOException, PermissionBackendException, ConfigInvalidException {
     try (TraceContext.TraceTimer ignored =
         TraceContext.newTimer(getClass().getSimpleName() + "#prepare", Metadata.empty())) {
-      requireNonNull(input.reviewer);
+      if (Strings.nullToEmpty(input.reviewer).trim().isEmpty()) {
+        return fail(input, FailureType.NOT_FOUND, "reviewer user identifier is required");
+      }
+
       boolean confirmed = input.confirmed();
       boolean allowByEmail =
           projectCache
diff --git a/java/com/google/gerrit/server/restapi/change/PostReviewers.java b/java/com/google/gerrit/server/restapi/change/PostReviewers.java
index b0e58c5..675610d 100644
--- a/java/com/google/gerrit/server/restapi/change/PostReviewers.java
+++ b/java/com/google/gerrit/server/restapi/change/PostReviewers.java
@@ -65,10 +65,6 @@
   public Response<ReviewerResult> apply(ChangeResource rsrc, ReviewerInput input)
       throws IOException, RestApiException, UpdateException, PermissionBackendException,
           ConfigInvalidException {
-    if (input.reviewer == null) {
-      throw new BadRequestException("missing reviewer field");
-    }
-
     ReviewerModification modification =
         reviewerModifier.prepare(rsrc.getNotes(), rsrc.getUser(), input, true);
     if (modification.op == null) {
diff --git a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
index 1b06b7b..3771bb9 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
@@ -59,6 +59,7 @@
 import com.google.gerrit.extensions.api.changes.ReviewInput.DraftHandling;
 import com.google.gerrit.extensions.api.changes.ReviewInput.RobotCommentInput;
 import com.google.gerrit.extensions.api.changes.ReviewResult;
+import com.google.gerrit.extensions.api.changes.ReviewerInput;
 import com.google.gerrit.extensions.client.ListChangesOption;
 import com.google.gerrit.extensions.client.ReviewerState;
 import com.google.gerrit.extensions.client.Side;
@@ -825,6 +826,41 @@
   }
 
   @Test
+  public void rejectAddingReviewerIfReviewerUserIdentifierIsMissing() throws Exception {
+    PushOneCommit.Result r = createChange();
+
+    // Add reviewer with ReviewerInput where the 'reviewer' field is not set.
+    ReviewerInput reviewerInput = new ReviewerInput();
+    reviewerInput.state = ReviewerState.REVIEWER;
+
+    ReviewInput reviewInput = ReviewInput.create();
+    reviewInput.reviewers = ImmutableList.of(reviewerInput);
+
+    ReviewResult reviewResult = gApi.changes().id(r.getChangeId()).current().review(reviewInput);
+    assertThat(reviewResult.error).isEqualTo("error adding reviewer");
+    assertThat(reviewResult.reviewers.keySet()).containsExactly(reviewerInput.reviewer);
+    assertThat(reviewResult.reviewers.get(reviewerInput.reviewer).error)
+        .isEqualTo("reviewer user identifier is required");
+
+    // Add reviewer with ReviewerInput where the 'reviewer' field is set to an empty string.
+    reviewerInput.reviewer = "";
+    reviewResult = gApi.changes().id(r.getChangeId()).current().review(reviewInput);
+    assertThat(reviewResult.error).isEqualTo("error adding reviewer");
+    assertThat(reviewResult.reviewers.keySet()).containsExactly(reviewerInput.reviewer);
+    assertThat(reviewResult.reviewers.get(reviewerInput.reviewer).error)
+        .isEqualTo("reviewer user identifier is required");
+
+    // Add reviewer with ReviewerInput where the 'reviewer' field is set to an empty string after
+    // trimming it.
+    reviewerInput.reviewer = "   ";
+    reviewResult = gApi.changes().id(r.getChangeId()).current().review(reviewInput);
+    assertThat(reviewResult.error).isEqualTo("error adding reviewer");
+    assertThat(reviewResult.reviewers.keySet()).containsExactly(reviewerInput.reviewer);
+    assertThat(reviewResult.reviewers.get(reviewerInput.reviewer).error)
+        .isEqualTo("reviewer user identifier is required");
+  }
+
+  @Test
   public void deletingReviewers() throws Exception {
     PushOneCommit.Result r = createChange();
 
diff --git a/javatests/com/google/gerrit/acceptance/rest/change/ChangeReviewersByEmailIT.java b/javatests/com/google/gerrit/acceptance/rest/change/ChangeReviewersByEmailIT.java
index 6cadf33..ca3a345 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/ChangeReviewersByEmailIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/ChangeReviewersByEmailIT.java
@@ -323,11 +323,19 @@
   }
 
   @Test
-  public void rejectMissingEmail() throws Exception {
+  public void rejectIfReviewerUserIdentifierIsMissing() throws Exception {
     PushOneCommit.Result r = createChange();
 
-    ReviewerResult result = gApi.changes().id(r.getChangeId()).addReviewer("");
-    assertThat(result.error).isEqualTo(" is not a valid user identifier");
+    ReviewerResult result = gApi.changes().id(r.getChangeId()).addReviewer((String) null);
+    assertThat(result.error).isEqualTo("reviewer user identifier is required");
+    assertThat(result.reviewers).isNull();
+
+    result = gApi.changes().id(r.getChangeId()).addReviewer("");
+    assertThat(result.error).isEqualTo("reviewer user identifier is required");
+    assertThat(result.reviewers).isNull();
+
+    result = gApi.changes().id(r.getChangeId()).addReviewer("   ");
+    assertThat(result.error).isEqualTo("reviewer user identifier is required");
     assertThat(result.reviewers).isNull();
   }