diff --git a/include/etl/optional.h b/include/etl/optional.h
index 657f5f92..5790727a 100644
--- a/include/etl/optional.h
+++ b/include/etl/optional.h
@@ -854,7 +854,8 @@ namespace etl
     //***************************************************************************
     /// Bool conversion operator.
     //***************************************************************************
-    ETL_CONSTEXPR14 operator bool() const
+    ETL_CONSTEXPR14 
+    ETL_EXPLICIT operator bool() const
     {
       return valid;
     }