diff --git a/.gitattributes b/.gitattributes
index 8b79287a21..b9d223f5fd 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -15341,6 +15341,7 @@ tests/webtbs/tw30524a.pp svneol=native#text/pascal
 tests/webtbs/tw30524b.pp svneol=native#text/pascal
 tests/webtbs/tw30530.pp svneol=native#text/pascal
 tests/webtbs/tw30534.pp svneol=native#text/pascal
+tests/webtbs/tw30535.pp svneol=native#text/pascal
 tests/webtbs/tw30537.pp svneol=native#text/pascal
 tests/webtbs/tw30552.pp svneol=native#text/pascal
 tests/webtbs/tw30570.pp svneol=native#text/plain
diff --git a/rtl/inc/heaptrc.pp b/rtl/inc/heaptrc.pp
index c029eb253d..29e403fc91 100644
--- a/rtl/inc/heaptrc.pp
+++ b/rtl/inc/heaptrc.pp
@@ -1446,9 +1446,14 @@ begin
 {$endif EXTRA}
   { if multithreading was initialized before heaptrc gets initialized (this is currently
     the case for windows dlls), then RelocateHeap gets never called and the lock
-    must be initialized already here
+    must be initialized already here,
+
+    however, IsMultithread is not set in this case on windows,
+    it is set only if a new thread is started
   }
+{$IfNDef WINDOWS}
   if IsMultithread then
+{$EndIf WINDOWS}
     TraceRelocateHeap;
 end;
 
diff --git a/rtl/win/systhrd.inc b/rtl/win/systhrd.inc
index 809c73ed72..80e85a3f63 100644
--- a/rtl/win/systhrd.inc
+++ b/rtl/win/systhrd.inc
@@ -141,7 +141,7 @@ var
 
     function SysRelocateThreadvar(offset : dword) : pointer; forward;
 
-    procedure SysInitMultithreading;
+    procedure SysInitTLS;
       begin
         { do not check IsMultiThread, as program could have altered it, out of Delphi habit }
 
@@ -151,8 +151,6 @@ var
            { We're still running in single thread mode, setup the TLS }
            TLSKey^:=TlsAlloc;
            InitThreadVars(@SysRelocateThreadvar);
-
-           IsMultiThread:=true;
          end;
       end;
 
@@ -250,7 +248,8 @@ var
         writeln('Creating new thread');
 {$endif DEBUG_MT}
         { Initialize multithreading if not done }
-        SysInitMultithreading;
+        SysInitTLS;
+        IsMultiThread:=true;
 
         { the only way to pass data to the newly created thread
           in a MT safe way, is to use the heap }
@@ -544,7 +543,7 @@ begin
 {$ifndef FPC_USE_TLS_DIRECTORY}
   if IsLibrary then
 {$endif}
-    SysInitMultithreading;
+    SysInitTLS;
 {$IFDEF SUPPORT_WIN95}
   { Try to find TryEnterCriticalSection function }
   KernelHandle:=GetModuleHandle(KernelDLL);
diff --git a/tests/webtbs/tw30535.pp b/tests/webtbs/tw30535.pp
new file mode 100644
index 0000000000..462f6f908f
--- /dev/null
+++ b/tests/webtbs/tw30535.pp
@@ -0,0 +1,10 @@
+{$ifdef unix}
+uses
+  cthreads;
+{$endif}
+
+begin
+  if IsMultiThread then
+    halt(1);
+  Writeln('ok');
+end.