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
@@ -91,6 +91,18 @@
       ->setNoDataString(
         pht('This build plan does not have any build steps yet.'));
 
+    // Check if all build steps are valid first.
+    $all_build_steps_valid = true;
+    foreach ($steps as $phid => $step) {
+      try {
+        $step->getStepImplementation();
+        continue;
+      } catch (Exception $ex) {
+        $all_build_steps_valid = false;
+        break;
+      }
+    }
+
     $i = 1;
     $last_depth = 0;
     $has_any_conflicts = false;
@@ -129,13 +141,23 @@
         continue;
       }
 
-      $item->addAttribute($implementation->getDescription());
-      $item->setHref($view_uri);
+      if (!$all_build_steps_valid) {
+        // 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 because steps are invalid.'));
+        continue;
+      }
 
       $depends = $step->getStepImplementation()->getDependencies($step);
       $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 =