diff --git a/src/applications/harbormaster/controller/HarbormasterPlanViewController.php b/src/applications/harbormaster/controller/HarbormasterPlanViewController.php
--- a/src/applications/harbormaster/controller/HarbormasterPlanViewController.php
+++ b/src/applications/harbormaster/controller/HarbormasterPlanViewController.php
@@ -129,13 +129,24 @@
         continue;
       }
 
-      $item->addAttribute($implementation->getDescription());
-      $item->setHref($view_uri);
-
-      $depends = $step->getStepImplementation()->getDependencies($step);
+      try {
+        $depends = $step->getStepImplementation()->getDependencies($step);
+      } catch (Exception $ex) {
+        // We can't calculate dependencies if another plan is missing
+        // an implementation.  Handle this scenario gracefully.
+        $item
+          ->setStatusIcon('fa-warning red')
+          ->addAttribute(pht(
+            'Unable to calculate dependencies for this build step. '.
+            'Are other build steps invalid?'));
+        continue;
+      }
       $inputs = $step->getStepImplementation()->getArtifactInputs();
       $outputs = $step->getStepImplementation()->getArtifactOutputs();
 
+      $item->addAttribute($implementation->getDescription());
+      $item->setHref($view_uri);
+
       $has_conflicts = false;
       if ($depends || $inputs || $outputs) {
         $available_artifacts =