Index: src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
===================================================================
--- src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
+++ src/applications/drydock/blueprint/DrydockAmazonEC2HostBlueprintImplementation.php
@@ -60,6 +60,14 @@
       $lease->getAttributes(),
       $this->getDetail('attributes'));
 
+    if ($lease->getAttribute('platform') === null) {
+      // If the platform is not specified, then we can lease against any
+      // existing host resource (this is used for working copies where
+      // the platform is not known ahead of time, but the target host
+      // resource ID is).
+      $platform_match = true;
+    }
+
     if ($platform_match && $custom_match) {
       $this->log(pht(
         'This blueprint can allocate the specified lease.'));
@@ -974,6 +982,10 @@
     }
 
     $lease->setAttribute('path', $full_path);
+    if ($lease->getAttribute('platform') === null) {
+      // If the lease didn't specify a platform, set it now.
+      $lease->setAttribute('platform', $platform);
+    }
 
     $this->log(pht(
       'Lease %d acquired on resource %d',