diff --git a/libc/stdio/stdio.cpp b/libc/stdio/stdio.cpp
index 1c31a27..5df1bb9 100644
--- a/libc/stdio/stdio.cpp
+++ b/libc/stdio/stdio.cpp
@@ -90,7 +90,7 @@
 
 class ScopedFileLock {
  public:
-  ScopedFileLock(FILE* fp) : fp_(fp) {
+  explicit ScopedFileLock(FILE* fp) : fp_(fp) {
     FLOCKFILE(fp_);
   }
   ~ScopedFileLock() {