diff --git a/libc/bionic/sigismember.cpp b/libc/bionic/sigismember.cpp
index 0dc73ac..16acd09 100644
--- a/libc/bionic/sigismember.cpp
+++ b/libc/bionic/sigismember.cpp
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <errno.h>
 #include <signal.h>
 
 int sigismember(const sigset_t* set, int signum) {