diff --git a/taichi/ir/ir.h b/taichi/ir/ir.h
index 7b48b949df0c7..14bfc11c2611a 100644
--- a/taichi/ir/ir.h
+++ b/taichi/ir/ir.h
@@ -511,7 +511,7 @@ class Block : public IRNode {
   // variables, and AllocaStmt for other variables.
   std::map<Identifier, Stmt *> local_var_to_stmt;
 
-  Block(Kernel *kernel = nullptr) {
+  explicit Block(Kernel *kernel = nullptr) {
     parent_ = kernel;
   }