diff --git a/app/models/Project.java b/app/models/Project.java
index 4561ef0ef..76a06703a 100644
--- a/app/models/Project.java
+++ b/app/models/Project.java
@@ -609,14 +609,16 @@ public void delete() {
             label.update();
         }
 
-        for(IssueLabelCategory category : IssueLabelCategory.findByProject(this)) {
-            category.delete();
-        }
-
+        // Issues must be deleted before issue labels because issues may refer
+        // issue labels.
         for(Issue issue : issues) {
             issue.delete();
         }
 
+        for(IssueLabelCategory category : IssueLabelCategory.findByProject(this)) {
+            category.delete();
+        }
+
         for (Assignee assignee : assignees) {
             assignee.delete();
         }