diff --git a/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterLeaseWorkingCopyBuildStepImplementation.php
@@ -135,6 +135,9 @@
     }
 
     $also_phids = $build_target->getFieldValue('repositoryPHIDs');
+    if (!is_array($also_phids)) {
+      $also_phids = array();
+    }
 
     $all_phids = $also_phids;
     $all_phids[] = $repository_phid;