diff --git a/src/agent.c b/src/agent.c
index 04f0523..9ff4ebb 100644
--- a/src/agent.c
+++ b/src/agent.c
@@ -124,10 +124,6 @@ struct agent_ops {
     agent_disconnect_func disconnect;
 };
 
-#ifndef INVALID_SOCKET
-#define INVALID_SOCKET -1
-#endif
-
 struct _LIBSSH2_AGENT
 {
     LIBSSH2_SESSION *session;  /* the session this "belongs to" */
diff --git a/src/libssh2_priv.h b/src/libssh2_priv.h
index 140e452..3d03807 100644
--- a/src/libssh2_priv.h
+++ b/src/libssh2_priv.h
@@ -154,6 +154,7 @@ static inline int writev(int sock, struct iovec *iov, int nvecs)
 typedef SOCKET libssh2_socket_t;
 #else /* !WIN32 */
 typedef int libssh2_socket_t;
+#define INVALID_SOCKET -1
 #endif /* WIN32 */
 
 /* RFC4253 section 6.1 Maximum Packet Length says: