diff --git a/core/gui/prioritize_dialog.py b/core/gui/prioritize_dialog.py
index d68c6b7d..d458f6cb 100644
--- a/core/gui/prioritize_dialog.py
+++ b/core/gui/prioritize_dialog.py
@@ -61,6 +61,8 @@ class PrioritizeDialog:
 
     def add_selected(self):
         # Add selected criteria in criteria_list to prioritization_list.
+        if self.criteria_list.selected_index is None:
+            return
         crit = self.criteria[self.criteria_list.selected_index]
         self.prioritizations.append(crit)
         self.prioritization_list[:] = [crit.display for crit in self.prioritizations]
diff --git a/core/tests/prioritize_test.py b/core/tests/prioritize_test.py
index 440d5183..ac3a0365 100644
--- a/core/tests/prioritize_test.py
+++ b/core/tests/prioritize_test.py
@@ -123,6 +123,11 @@ def test_remove_crit_from_list(app):
     ]
     eq_(app.pdialog.prioritization_list[:], expected)
 
+@with_app(app_normal_results)
+def test_add_crit_without_selection(app):
+    # Adding a criterion without having made a selection doesn't cause a crash.
+    app.pdialog.add_selected() # no crash
+
 #---
 def app_one_name_ends_with_number():
     dupes = [