diff --git a/src/unistd/isatty.c b/src/unistd/isatty.c
index c8badaf55edd7d6c4e692e5d238e093848cc7290..75a9c186a941896f90dd0985fe4b1fe86d351f54 100644
--- a/src/unistd/isatty.c
+++ b/src/unistd/isatty.c
@@ -1,9 +1,13 @@
 #include <unistd.h>
+#include <errno.h>
 #include <sys/ioctl.h>
 #include "syscall.h"
 
 int isatty(int fd)
 {
 	struct winsize wsz;
-	return !__syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz);
+	unsigned long r = syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz);
+	if (r == 0) return 1;
+	if (errno != EBADF) errno = ENOTTY;
+	return 0;
 }
diff --git a/src/unistd/ttyname_r.c b/src/unistd/ttyname_r.c
index cb364c2913b442934304ac95d8fea48f01b80052..82acb75e19d60a6a075e7398dc23d9b4f8729e5d 100644
--- a/src/unistd/ttyname_r.c
+++ b/src/unistd/ttyname_r.c
@@ -9,7 +9,7 @@ int ttyname_r(int fd, char *name, size_t size)
 	char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
 	ssize_t l;
 
-	if (!isatty(fd)) return ENOTTY;
+	if (!isatty(fd)) return errno;
 
 	__procfdname(procname, fd);
 	l = readlink(procname, name, size);