diff --git a/Userland/Libraries/LibC/stdio.cpp b/Userland/Libraries/LibC/stdio.cpp
index 3e75b859fe..b12c547e5d 100644
--- a/Userland/Libraries/LibC/stdio.cpp
+++ b/Userland/Libraries/LibC/stdio.cpp
@@ -30,6 +30,7 @@ public:
         : m_fd(fd)
         , m_mode(mode)
     {
+        __pthread_mutex_init(&m_mutex, nullptr);
     }
     ~FILE();