diff --git a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
index d070966..131dbf9 100644
--- a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
+++ b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
@@ -46,6 +46,7 @@
 import static javax.servlet.http.HttpServletResponse.SC_NO_CONTENT;
 import static javax.servlet.http.HttpServletResponse.SC_OK;
 import static javax.servlet.http.HttpServletResponse.SC_PRECONDITION_FAILED;
+import static javax.servlet.http.HttpServletResponse.SC_SERVICE_UNAVAILABLE;
 
 import com.google.common.base.CharMatcher;
 import com.google.common.base.Joiner;
@@ -102,9 +103,11 @@
 import com.google.gerrit.server.audit.AuditService;
 import com.google.gerrit.server.audit.ExtendedHttpAuditEvent;
 import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.git.LockFailureException;
 import com.google.gerrit.server.permissions.GlobalPermission;
 import com.google.gerrit.server.permissions.PermissionBackend;
 import com.google.gerrit.server.permissions.PermissionBackendException;
+import com.google.gerrit.server.update.UpdateException;
 import com.google.gerrit.util.http.RequestUtil;
 import com.google.gson.ExclusionStrategy;
 import com.google.gson.FieldAttributes;
@@ -480,6 +483,15 @@
     } catch (NotImplementedException e) {
       responseBytes =
           replyError(req, res, status = SC_NOT_IMPLEMENTED, messageOr(e, "Not Implemented"), e);
+    } catch (UpdateException e) {
+      Throwable t = e.getCause();
+      if (t instanceof LockFailureException) {
+        responseBytes =
+            replyError(req, res, status = SC_SERVICE_UNAVAILABLE, messageOr(t, "Lock failure"), e);
+      } else {
+        status = SC_INTERNAL_SERVER_ERROR;
+        responseBytes = handleException(e, req, res);
+      }
     } catch (Exception e) {
       status = SC_INTERNAL_SERVER_ERROR;
       responseBytes = handleException(e, req, res);