diff --git a/gen/function-inlining.cpp b/gen/function-inlining.cpp
index 37c64e1363..4773067235 100644
--- a/gen/function-inlining.cpp
+++ b/gen/function-inlining.cpp
@@ -30,7 +30,7 @@ struct MoreThanXStatements : public StoppableVisitor {
   /// The statement count.
   unsigned count;
 
-  MoreThanXStatements(unsigned X) : threshold(X), count(0) {}
+  explicit MoreThanXStatements(unsigned X) : threshold(X), count(0) {}
 
   using StoppableVisitor::visit;