diff --git a/src/addsub/s_fp_add.c b/src/addsub/s_fp_add.c
index 4822be3..47406de 100644
--- a/src/addsub/s_fp_add.c
+++ b/src/addsub/s_fp_add.c
@@ -16,7 +16,7 @@ void s_fp_add(fp_int *a, fp_int *b, fp_int *c)
   register fp_word  t;
 
   y       = MAX(a->used, b->used);
-  oldused = c->used;
+  oldused = MIN(c->used, FP_SIZE);
   c->used = y;
  
   t = 0;