diff --git a/usr.bin/comm/comm.c b/usr.bin/comm/comm.c
--- a/usr.bin/comm/comm.c
+++ b/usr.bin/comm/comm.c
@@ -172,6 +172,8 @@
 				(void)printf("%s%s\n", col2, line2);
 		}
 	}
+	if (ferror(stdout) != 0 || fflush(stdout) != 0)
+		err(1, "stdout");
 	exit(0);
 }