diff --git a/README.md b/README.md
index 27274ed..2e826fe 100644
--- a/README.md
+++ b/README.md
@@ -256,6 +256,8 @@ struct blackboard_type {
 };
 ```
 
+> **NB:** The blackboard needs to be comparable (`a == b`) and hashable.
+
 Next, create a class for each action that you want to be able to perform:
 
 ```cpp
diff --git a/include/aitoolkit/goap.hpp b/include/aitoolkit/goap.hpp
index 628a747..10049bd 100644
--- a/include/aitoolkit/goap.hpp
+++ b/include/aitoolkit/goap.hpp
@@ -50,6 +50,8 @@ struct blackboard_type {
 };
 ```
 
+> **NB:** The blackboard needs to be comparable (`a == b`) and hashable.
+
 Next, create a class for each action that you want to be able to perform:
 
 ```cpp