diff --git a/resources/sql/autopatches/20190129.project.01.spaces.php b/resources/sql/autopatches/20190129.project.01.spaces.php
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20190129.project.01.spaces.php
@@ -0,0 +1,18 @@
+<?php
+
+// See PHI1046. The "spacePHID" column for milestones may have fallen out of
+// sync; correct all existing values.
+
+$table = new PhabricatorProject();
+$conn = $table->establishConnection('w');
+$table_name = $table->getTableName();
+
+foreach (new LiskRawMigrationIterator($conn, $table_name) as $project_row) {
+  queryfx(
+    $conn,
+    'UPDATE %R SET spacePHID = %ns
+      WHERE parentProjectPHID = %s AND milestoneNumber IS NOT NULL',
+    $table,
+    $project_row['spacePHID'],
+    $project_row['phid']);
+}
diff --git a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
--- a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
+++ b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
@@ -249,6 +249,17 @@
         ->rematerialize($new_parent);
     }
 
+    // See PHI1046. Milestones are always in the Space of their parent project.
+    // Synchronize the database values to match the application values.
+    $conn = $object->establishConnection('w');
+    queryfx(
+      $conn,
+      'UPDATE %R SET spacePHID = %ns
+        WHERE parentProjectPHID = %s AND milestoneNumber IS NOT NULL',
+      $object,
+      $object->getSpacePHID(),
+      $object->getPHID());
+
     return parent::applyFinalEffects($object, $xactions);
   }