diff --git a/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php b/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
--- a/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
+++ b/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php
@@ -559,8 +559,7 @@
       return false;
     }
 
-    // TODO: For now, this is only supported for Git.
-    if (!$repository->isGit()) {
+    if (!$repository->supportsSynchronization()) {
       return false;
     }
 
diff --git a/src/applications/repository/storage/PhabricatorRepository.php b/src/applications/repository/storage/PhabricatorRepository.php
--- a/src/applications/repository/storage/PhabricatorRepository.php
+++ b/src/applications/repository/storage/PhabricatorRepository.php
@@ -1929,10 +1929,31 @@
           'Cluster hosts must correctly route their intracluster requests.'));
     }
 
+    if (count($results) > 1) {
+      if (!$this->supportsSynchronization()) {
+        throw new Exception(
+          pht(
+            'Repository "%s" is bound to multiple active repository hosts, '.
+            'but this repository does not support cluster synchronization. '.
+            'Declusterize this repository or move it to a service with only '.
+            'one host.',
+            $this->getDisplayName()));
+      }
+    }
+
     shuffle($results);
     return head($results);
   }
 
+  public function supportsSynchronization() {
+    // TODO: For now, this is only supported for Git.
+    if (!$this->isGit()) {
+      return false;
+    }
+
+    return true;
+  }
+
   public function getAlmanacServiceCacheKey() {
     $service_phid = $this->getAlmanacServicePHID();
     if (!$service_phid) {