diff --git a/spot/tl/formula.hh b/spot/tl/formula.hh
index 57cc8f91c..f355a4d23 100644
--- a/spot/tl/formula.hh
+++ b/spot/tl/formula.hh
@@ -842,7 +842,7 @@ namespace spot
       return ptr_ != nullptr;
     }
 
-    operator bool() const noexcept
+    explicit operator bool() const noexcept
     {
       return ptr_ != nullptr;
     }