diff --git a/impl_generator.cpp b/impl_generator.cpp
index c19234f..42229bd 100644
--- a/impl_generator.cpp
+++ b/impl_generator.cpp
@@ -13,6 +13,8 @@ int ImplGenerator::get_next_num()
     }
     while (already_generated_numbers.count(num) > 0);
     
+    already_generated_numbers.emplace(num);
+
     internal_counter++;
 
     return num;