diff --git a/tests/sys_statvfs_test.cpp b/tests/sys_statvfs_test.cpp
index 6b19e13..bff9e20 100644
--- a/tests/sys_statvfs_test.cpp
+++ b/tests/sys_statvfs_test.cpp
@@ -30,6 +30,11 @@
   EXPECT_EQ(0U, sb.f_ffree);
   EXPECT_EQ(0U, sb.f_fsid);
   EXPECT_EQ(255U, sb.f_namemax);
+
+  // The kernel sets a private bit to indicate that f_flags is valid.
+  // This flag is not supposed to be exposed to libc clients.
+  static const uint32_t ST_VALID = 0x0020;
+  EXPECT_TRUE((sb.f_flag & ST_VALID) == 0) << sb.f_flag;
 }
 
 TEST(sys_statvfs, statvfs) {
@@ -51,6 +56,7 @@
   close(fd);
   Check(sb);
 }
+
 TEST(sys_statvfs, fstatvfs64) {
   struct statvfs64 sb;
   int fd = open("/proc", O_RDONLY);
diff --git a/tests/sys_vfs_test.cpp b/tests/sys_vfs_test.cpp
index 4b05660..a521967 100644
--- a/tests/sys_vfs_test.cpp
+++ b/tests/sys_vfs_test.cpp
@@ -31,6 +31,11 @@
   EXPECT_EQ(0, sb.f_fsid.__val[0]);
   EXPECT_EQ(0, sb.f_fsid.__val[1]);
   EXPECT_EQ(255, static_cast<int>(sb.f_namelen));
+
+  // The kernel sets a private bit to indicate that f_flags is valid.
+  // This flag is not supposed to be exposed to libc clients.
+  static const uint32_t ST_VALID = 0x0020;
+  EXPECT_TRUE((sb.f_flags & ST_VALID) == 0) << sb.f_flags;
 }
 
 TEST(sys_vfs, statfs) {
@@ -52,6 +57,7 @@
   close(fd);
   Check(sb);
 }
+
 TEST(sys_vfs, fstatfs64) {
   struct statfs64 sb;
   int fd = open("/proc", O_RDONLY);