diff --git a/tasks.c b/tasks.c
index bbd0b0207..e4c3deaac 100644
--- a/tasks.c
+++ b/tasks.c
@@ -3592,7 +3592,8 @@ 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 );
@@ -3611,7 +3612,8 @@ 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 ];