diff --git a/test/unistd/dup.c b/test/unistd/dup.c
index 418823f020e9e..7447ac46dcb2b 100644
--- a/test/unistd/dup.c
+++ b/test/unistd/dup.c
@@ -22,6 +22,9 @@ int main() {
   f2 = open("/", O_RDONLY);
   f3 = dup(f);
   printf("errno: %d\n", errno);
+  assert(f != -1);
+  assert(f2 != -1);
+  assert(f3 != -1);
   printf("f: %d\n", f != f2 && f != f3);
   printf("f2,f3: %d\n", f2 != f3);
   printf("close(f1): %d\n", close(f));
@@ -34,6 +37,9 @@ int main() {
   f = open("/", O_RDONLY);
   f2 = open("/", O_RDONLY);
   f3 = dup2(f, f2);
+  assert(f != -1);
+  assert(f2 != -1);
+  assert(f3 != -1);
   printf("errno: %d\n", errno);
   printf("f: %d\n", f != f2 && f != f3);
   printf("f2,f3: %d\n", f2 == f3);
@@ -45,7 +51,8 @@ int main() {
 
   printf("DUP2 bad fds\n");
   f = dup2(-2, -2);
-  printf("f: %d\n", f == -1);
+  printf("f: %d\n", f);
+  assert(f == -1);
   printf("errno: %d\n", errno);
   printf("close(f): %d\n", close(f));
   printf("\n");
@@ -53,11 +60,14 @@ int main() {
 
   printf("DUP2 bad newfd\n");
   f = open("/", O_RDONLY);
+  assert(f != -1);
   f3 = dup2(f, -1);
   printf("f3: %d\n", f3);
+  assert(f3 == -1);
   printf("errno: %d\n", errno);
   f3 = dup2(f, 256000);
   printf("f3: %d\n", f3);
+  assert(f3 == -1);
   printf("errno: %d\n", errno);
   printf("close(f1): %d\n", close(f));
   printf("\n");
@@ -67,6 +77,7 @@ int main() {
   int p[2];
   pipe(p);
   int g = dup2(p[0], 7);
+  assert(g != -1);
   int rtn = write(p[1], "abc", 3);
   assert(rtn == 3);
   char buf[5] = {0};
@@ -79,6 +90,8 @@ int main() {
   printf("DUP shared seek position\n");
   f = open("./blah.txt", O_RDWR | O_CREAT | O_EXCL, 0600);
   f2 = dup(f);
+  assert(f != -1);
+  assert(f2 != -1);
   rtn = write(f2, "abcabc\n", 7);
   assert(rtn == 7);
   assert(lseek(f, 0, SEEK_CUR) == 7);
@@ -90,6 +103,8 @@ int main() {
   printf("DUP truncate\n");
   f = open("./blah.txt", O_RDWR, 0600);
   f2 = dup(f);
+  assert(f != -1);
+  assert(f2 != -1);
   rtn = ftruncate(f2, 0);
   assert(rtn == 0);
   assert(lseek(f, 0, SEEK_END) == 0);
diff --git a/test/unistd/dup.out b/test/unistd/dup.out
index b8c276f7528bf..ed3f03e4d48d6 100644
--- a/test/unistd/dup.out
+++ b/test/unistd/dup.out
@@ -15,7 +15,7 @@ close(f2): 0
 close(f3): -1
 
 DUP2 bad fds
-f: 1
+f: -1
 errno: 8
 close(f): -1