diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index 46919a6..7332146 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -4938,6 +4938,7 @@ defined as defaulted.
 
     class AbstractBase {
     public:
+        virtual void foo() = 0;  // at least one abstract method to make the class abstract
         virtual ~AbstractBase() = default;
         // ...
     };