diff --git a/third_party/vaca/src/unix/MutexImpl.h b/third_party/vaca/src/unix/MutexImpl.h
index d3d1b5ff8..3d8ebc075 100644
--- a/third_party/vaca/src/unix/MutexImpl.h
+++ b/third_party/vaca/src/unix/MutexImpl.h
@@ -30,6 +30,7 @@
 // OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <pthread.h>
+#include <errno.h>
 
 class Vaca::Mutex::MutexImpl
 {