diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index bd84d30..9071acf 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -59,7 +59,7 @@
 
 class ChildGuard {
  public:
-  ChildGuard(pid_t pid) : pid(pid) {}
+  explicit ChildGuard(pid_t pid) : pid(pid) {}
 
   ~ChildGuard() {
     kill(pid, SIGKILL);