diff --git a/simplecpp.cpp b/simplecpp.cpp
index 4b24e311..8939ff2e 100644
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -1771,7 +1771,7 @@ namespace simplecpp {
         };
 
         struct invalidDirectiveAsMacroParameter : public Error {
-            invalidDirectiveAsMacroParameter(const Location &loc)
+            explicit invalidDirectiveAsMacroParameter(const Location &loc)
                 : Error(loc, "it is invalid to use a preprocessor directive as macro parameter") {}
         };