diff --git a/rts/Arena.c b/rts/Arena.c
index a4ff11be7d675d29c002337ac19cd5e19188801d..1d6dac623c37b228657140165443adf063b388c2 100644
--- a/rts/Arena.c
+++ b/rts/Arena.c
@@ -82,7 +82,7 @@ arenaAlloc( Arena *arena, size_t size )
         // allocate a fresh block...
         req_blocks =  (W_)BLOCK_ROUND_UP(size) / BLOCK_SIZE;
         bd = allocGroup_lock(req_blocks);
-        arena_blocks += req_blocks;
+        arena_blocks += bd->blocks;
 
         bd->gen_no  = 0;
         bd->gen     = NULL;