diff --git a/libc/bionic/sysconf.cpp b/libc/bionic/sysconf.cpp
index 2442fd9..2b3200c 100644
--- a/libc/bionic/sysconf.cpp
+++ b/libc/bionic/sysconf.cpp
@@ -221,9 +221,7 @@
     case _SC_LEVEL4_CACHE_LINESIZE:   return 0;
 
     default:
-      // Posix says EINVAL is the only error that shall be returned,
-      // but glibc uses ENOSYS.
-      errno = ENOSYS;
+      errno = EINVAL;
       return -1;
   }
 }
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index db4859b..fd99455 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -836,19 +836,23 @@
 #endif // defined(__BIONIC__)
 }
 
-#define VERIFY_SYSCONF_UNSUPPORTED(name) VerifySysconf(name, #name, [](long v){return v == -1;})
+#define VERIFY_SYSCONF_UNKNOWN(name) \
+  VerifySysconf(name, #name, [](long v){return v == -1 && errno == EINVAL;})
+
+#define VERIFY_SYSCONF_UNSUPPORTED(name) \
+  VerifySysconf(name, #name, [](long v){return v == -1 && errno == 0;})
 
 // sysconf() means unlimited when it returns -1 with errno unchanged.
 #define VERIFY_SYSCONF_POSITIVE(name) \
-  VerifySysconf(name, #name, [](long v){return (v > 0 || v == -1);})
+  VerifySysconf(name, #name, [](long v){return (v > 0 || v == -1) && errno == 0;})
 
 #define VERIFY_SYSCONF_POSIX_VERSION(name) \
-  VerifySysconf(name, #name, [](long v){return v == _POSIX_VERSION;})
+  VerifySysconf(name, #name, [](long v){return v == _POSIX_VERSION && errno == 0;})
 
 static void VerifySysconf(int option, const char *option_name, bool (*verify)(long)) {
   errno = 0;
   long ret = sysconf(option);
-  EXPECT_TRUE(0 == errno && verify(ret)) << "name = " << option_name << ", ret = "
+  EXPECT_TRUE(verify(ret)) << "name = " << option_name << ", ret = "
       << ret <<", Error Message: " << strerror(errno);
 }
 
@@ -879,17 +883,17 @@
   VERIFY_SYSCONF_POSITIVE(_SC_RE_DUP_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_STREAM_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_TZNAME_MAX);
-  VerifySysconf(_SC_XOPEN_VERSION, "_SC_XOPEN_VERSION", [](long v){return v == _XOPEN_VERSION;});
+  VerifySysconf(_SC_XOPEN_VERSION, "_SC_XOPEN_VERSION", [](long v){return v == _XOPEN_VERSION && errno == 0;});
   VERIFY_SYSCONF_POSITIVE(_SC_ATEXIT_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_IOV_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_PAGESIZE);
   VERIFY_SYSCONF_POSITIVE(_SC_PAGE_SIZE);
   VerifySysconf(_SC_PAGE_SIZE, "_SC_PAGE_SIZE",
-                [](long v){return v == sysconf(_SC_PAGESIZE) && v == getpagesize();});
+                [](long v){return v == sysconf(_SC_PAGESIZE) && errno == 0 && v == getpagesize();});
   VERIFY_SYSCONF_POSITIVE(_SC_XOPEN_UNIX);
   VERIFY_SYSCONF_POSITIVE(_SC_AIO_LISTIO_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_AIO_MAX);
-  VerifySysconf(_SC_AIO_PRIO_DELTA_MAX, "_SC_AIO_PRIO_DELTA_MAX", [](long v){return v >= 0;});
+  VerifySysconf(_SC_AIO_PRIO_DELTA_MAX, "_SC_AIO_PRIO_DELTA_MAX", [](long v){return v >= 0 && errno == 0;});
   VERIFY_SYSCONF_POSITIVE(_SC_DELAYTIMER_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_MQ_OPEN_MAX);
   VERIFY_SYSCONF_POSITIVE(_SC_MQ_PRIO_MAX);
@@ -1024,6 +1028,11 @@
   ASSERT_EQ(ARG_MAX, sysconf(_SC_ARG_MAX));
 }
 
+TEST(UNISTD_TEST, sysconf_unknown) {
+  VERIFY_SYSCONF_UNKNOWN(-1);
+  VERIFY_SYSCONF_UNKNOWN(666);
+}
+
 TEST(UNISTD_TEST, dup2_same) {
   // POSIX says of dup2:
   // If fildes2 is already a valid open file descriptor ...