diff --git a/libc/bionic/pthread_internal.cpp b/libc/bionic/pthread_internal.cpp
index 2b7a99a..c058384 100644
--- a/libc/bionic/pthread_internal.cpp
+++ b/libc/bionic/pthread_internal.cpp
@@ -43,7 +43,7 @@
 
 template <bool write> class ScopedRWLock {
  public:
-  ScopedRWLock(pthread_rwlock_t* rwlock) : rwlock_(rwlock) {
+  explicit ScopedRWLock(pthread_rwlock_t* rwlock) : rwlock_(rwlock) {
     (write ? pthread_rwlock_wrlock : pthread_rwlock_rdlock)(rwlock_);
   }