diff --git a/src/dll/posix.c b/src/dll/posix.c
index 73a77779..a72c0def 100644
--- a/src/dll/posix.c
+++ b/src/dll/posix.c
@@ -325,7 +325,7 @@ FSP_API NTSTATUS FspPosixMapSidToUid(PSID Sid, PUINT32 PUid)
          */
         *PUid = 0x60000 + Rid;
     }
-    else
+    else if (0 != Authority)
     {
         /* [IDMAP]
          * Other well-known SIDs:
diff --git a/tst/winfsp-tests/posix-test.c b/tst/winfsp-tests/posix-test.c
index 03c3df14..eacdea05 100644
--- a/tst/winfsp-tests/posix-test.c
+++ b/tst/winfsp-tests/posix-test.c
@@ -10,7 +10,7 @@ void posix_map_sid_test(void)
         UINT32 Uid;
     } map[] =
     {
-        { L"S-1-0-0", 0x10000 },
+        { L"S-1-0-0", -1 },
         { L"S-1-1-0", 0x10100 },
         { L"S-1-2-0", 0x10200 },
         { L"S-1-2-1", 0x10201 },