diff --git a/tasks.c b/tasks.c
index 8a7bf9875..2435d7051 100644
--- a/tasks.c
+++ b/tasks.c
@@ -3600,7 +3600,8 @@ static portTASK_FUNCTION( prvIdleTask, pvParameters )
     {
         TCB_t * pxTCB;
 
-        if( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS )
+        if( ( xIndex >= 0 ) &&
+            ( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS ) )
         {
             pxTCB = prvGetTCBFromHandle( xTaskToSet );
             configASSERT( pxTCB != NULL );
@@ -3619,7 +3620,8 @@ static portTASK_FUNCTION( prvIdleTask, pvParameters )
         void * pvReturn = NULL;
         TCB_t * pxTCB;
 
-        if( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS )
+        if( ( xIndex >= 0 ) &&
+            ( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS ) )
         {
             pxTCB = prvGetTCBFromHandle( xTaskToQuery );
             pvReturn = pxTCB->pvThreadLocalStoragePointers[ xIndex ];