Index: sys/kern/kern_sendfile.c
===================================================================
--- sys/kern/kern_sendfile.c
+++ sys/kern/kern_sendfile.c
@@ -1007,8 +1007,15 @@
 	    uap->nbytes, &sbytes, uap->flags, td);
 	fdrop(fp, td);
 
-	if (uap->sbytes != NULL)
-		copyout(&sbytes, uap->sbytes, sizeof(off_t));
+	/*
+	 * Only copy out the number of bytes if fo_sendfile did not fail or
+	 * failed with EAGAIN or EINTR.
+	 */
+	if (error == 0 || error == EAGAIN || error == EINTR) {
+		if (uap->sbytes != NULL) {
+			error = copyout(&sbytes, uap->sbytes, sizeof(off_t));
+		}
+	}
 
 out:
 	free(hdr_uio, M_IOV);