diff --git "a/dll/win32/winhttp/request.c" "b/dll/win32/winhttp/request.c"
index da28f7b1826..9867f873577 100644
--- "a/dll/win32/winhttp/request.c"
+++ "b/dll/win32/winhttp/request.c"
@@ -2989,7 +2989,7 @@ done:
         }
     }
 
-    if (ret && available) *available = count;
+    if (ret && count && available) *available = count;
     return ret;
 }
 
@@ -3034,7 +3034,11 @@ BOOL WINAPI WinHttpQueryDataAvailable( HINTERNET hrequest, LPDWORD available )
         ret = queue_task( (struct task_header *)q );
     }
     else
+    {
+        EnterCriticalSection( &request->task_cs );
         ret = query_data_available( request, available, FALSE );
+        LeaveCriticalSection( &request->task_cs );
+    }
 
     release_object( &request->hdr );
     if (ret) SetLastError( ERROR_SUCCESS );