diff --git a/libc/bionic/sigfillset.cpp b/libc/bionic/sigfillset.cpp
index 7b7cbb8..37792e5 100644
--- a/libc/bionic/sigfillset.cpp
+++ b/libc/bionic/sigfillset.cpp
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <errno.h>
 #include <signal.h>
 
 int sigfillset(sigset_t* set) {