diff --git a/src/thread.h b/src/thread.h
index d4cae360f68..64448007531 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -19,6 +19,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #ifndef THREAD_H
 #define THREAD_H
 
+#include <sys/types.h>		/* for ssize_t used by regex.h */
 #include "regex.h"
 
 #ifdef WINDOWSNT