diff --git a/rtl/inc/thread.inc b/rtl/inc/thread.inc
index fda06c3790..697a7b90da 100644
--- a/rtl/inc/thread.inc
+++ b/rtl/inc/thread.inc
@@ -71,7 +71,11 @@ Var
         { Open all stdio fds again }
         SysFlushStdio;
 {$endif FPC_HAS_FEATURE_CONSOLEIO}
-        CurrentTM.ReleaseThreadVars;
+        { Support platforms where threadvar memory is managed outside of the RTL:
+          reset ThreadID and allow ReleaseThreadVars to be unassigned }
+        ThreadID := 0;
+        if assigned(CurrentTM.ReleaseThreadVars) then
+          CurrentTM.ReleaseThreadVars;
       end;
 
 {*****************************************************************************
diff --git a/rtl/win/systhrd.inc b/rtl/win/systhrd.inc
index 58d6f14dfb..91a16f93e6 100644
--- a/rtl/win/systhrd.inc
+++ b/rtl/win/systhrd.inc
@@ -121,10 +121,14 @@ var
         { these aren't allocated yet ...            }
         { allocate room on the heap for the thread vars }
         errorsave:=GetLastError;
+        if tlskey=$ffffffff then
+          RunError(226);
         dataindex:=TlsGetValue(tlskey);
         if dataindex=nil then
           begin
             dataindex:=pointer(LocalAlloc(LMEM_FIXED or LMEM_ZEROINIT,threadvarblocksize));
+            if dataindex=nil then
+              RunError(226);
             TlsSetValue(tlskey,dataindex);
           end;
         SetLastError(errorsave);
@@ -150,11 +154,9 @@ var
 
     procedure SysFiniMultithreading;
       begin
-        if IsMultiThread then
-          begin
-            TlsFree(TLSKey);
-            TLSKey:=$ffffffff;
-          end;
+        if TLSKey<>$ffffffff then
+          TlsFree(TLSKey);
+        TLSKey:=$ffffffff;
       end;
 
     function SysRelocateThreadvar(offset : dword) : pointer;
@@ -176,9 +178,16 @@ var
 
 
     procedure SysReleaseThreadVars;
+      var
+        p: pointer;
       begin
-        LocalFree(TlsGetValue(tlskey));
-        TlsSetValue(tlskey, nil);
+        if TLSKey<>$ffffffff then
+          begin
+            p:=TlsGetValue(tlskey);
+            if Assigned(p) then
+              LocalFree(p);
+            TlsSetValue(tlskey, nil);
+          end;
       end;
 
 
diff --git a/rtl/win/syswin.inc b/rtl/win/syswin.inc
index 12d727c99e..b34e50b6ca 100644
--- a/rtl/win/syswin.inc
+++ b/rtl/win/syswin.inc
@@ -30,7 +30,7 @@ function Dll_entry{$ifdef FPC_HAS_INDIRECT_MAIN_INFORMATION}(const info : TEntry
      EntryInformation:=info;
 {$endif FPC_HAS_INDIRECT_MAIN_INFORMATION}
      IsLibrary:=true;
-     Dll_entry:=false;
+     Dll_entry:=false;  { return value is ignored, except when DLLreason=DLL_PROCESS_ATTACH }
      case DLLreason of
        DLL_PROCESS_ATTACH :
          begin
@@ -61,7 +61,6 @@ function Dll_entry{$ifdef FPC_HAS_INDIRECT_MAIN_INFORMATION}(const info : TEntry
 
            if assigned(Dll_Thread_Attach_Hook) then
              Dll_Thread_Attach_Hook(DllParam);
-           Dll_entry:=true; { return value is ignored }
         end;
        DLL_THREAD_DETACH :
          begin
@@ -70,11 +69,9 @@ function Dll_entry{$ifdef FPC_HAS_INDIRECT_MAIN_INFORMATION}(const info : TEntry
            { Release Threadvars }
            if TlsGetValue(TLSKey)<>nil then
              DoneThread; { Assume everything is idempotent there }
-           Dll_entry:=true; { return value is ignored }
          end;
        DLL_PROCESS_DETACH :
          begin
-           Dll_entry:=true; { return value is ignored }
 	   if MainThreadIDWin32=0 then // already been here.
 	     exit;
            If SetJmp(DLLBuf) = 0 then
@@ -82,7 +79,7 @@ function Dll_entry{$ifdef FPC_HAS_INDIRECT_MAIN_INFORMATION}(const info : TEntry
            if assigned(Dll_Process_Detach_Hook) then
              Dll_Process_Detach_Hook(DllParam);
 
-           DoneThread;
+           SysReleaseThreadVars;
            { Free TLS resources used by ThreadVars }
            SysFiniMultiThreading;
            MainThreadIDWin32:=0;