diff --git a/lib/otp/otp_locl.h b/lib/otp/otp_locl.h
index 6852fd95a..e34647272 100644
--- a/lib/otp/otp_locl.h
+++ b/lib/otp/otp_locl.h
@@ -63,6 +63,8 @@
 #include <io.h>
 #endif
 
+#include <protos.h>
+
 #include <otp.h>
 
 #include <xdbm.h>