diff --git a/lib/tlsf/src/tlsf.c b/lib/tlsf/src/tlsf.c
index cea53c3eb8..46ae7616b1 100644
--- a/lib/tlsf/src/tlsf.c
+++ b/lib/tlsf/src/tlsf.c
@@ -338,14 +338,14 @@ static __inline__ void MAPPING_INSERT(size_t _r, int *_fl, int *_sl)
 
 static __inline__ bhdr_t *FIND_SUITABLE_BLOCK(tlsf_t * _tlsf, int *_fl, int *_sl)
 {
-    u32_t _tmp = _tlsf->sl_bitmap[*_fl] & (~0 << *_sl);
+    u32_t _tmp = _tlsf->sl_bitmap[*_fl] & (~0u << *_sl);
     bhdr_t *_b = NULL;
 
     if (_tmp) {
         *_sl = ls_bit(_tmp);
         _b = _tlsf->matrix[*_fl][*_sl];
     } else {
-        *_fl = ls_bit(_tlsf->fl_bitmap & (~0 << (*_fl + 1)));
+        *_fl = ls_bit(_tlsf->fl_bitmap & (~0u << (*_fl + 1)));
         if (*_fl > 0) {         /* likely */
             *_sl = ls_bit(_tlsf->sl_bitmap[*_fl]);
             _b = _tlsf->matrix[*_fl][*_sl];