diff --git a/runtime.c b/runtime.c
index 1e752d48..168a276a 100644
--- a/runtime.c
+++ b/runtime.c
@@ -2464,21 +2464,21 @@ object C_bignum_simplify(object big)
 
   switch(length) {
   case 0:
-    //if (C_in_scratchspacep(C_internal_bignum_vector(big)))
-    //  C_mutate_scratch_slot(NULL, C_internal_bignum_vector(big));
     return obj_int2obj(0);
   case 1:
     tmp = *start;
-    if (C_bignum_negativep(big) ?
-        !(tmp & C_INT_SIGN_BIT) && C_fitsinfixnump(-(C_word)tmp) :
-        C_ufitsinfixnump(tmp)) {
-      if (C_in_scratchspacep(C_internal_bignum_vector(big)))
-        C_mutate_scratch_slot(NULL, C_internal_bignum_vector(big));
-      return C_bignum_negativep(big) ? C_fix(-(C_word)tmp) : C_fix(tmp);
+    if (((uint32_t)tmp) <= CYC_FIXNUM_MAX) {
+      if (C_bignum_negativep(big)) {
+        return obj_int2obj(-(int32_t)tmp);
+      } else {
+        return obj_int2obj(tmp);
+      }
     }
     /* FALLTHROUGH */
   default:
-    if (scan < last_digit) C_bignum_mutate_size(big, length);
+    if (scan < last_digit) {
+      C_bignum_size(big) = length;
+    }
     return big;
   }
 }
@@ -2761,8 +2761,7 @@ object bignum2_plus_unsigned(void *data, bignum2_type *x, bignum2_type *y, int n
   }
   assert(scan_r <= end_r);
 
-  // TODO: return C_bignum_simplify(result);
-  return result; // TODO: no, could be a fixnum. need to simplify using above!
+  return C_bignum_simplify(result);
 }
 
 object Cyc_symbol2string(void *data, object cont, object sym)
diff --git a/test-bn.scm b/test-bn.scm
index caf2e4e0..183bfacd 100644
--- a/test-bn.scm
+++ b/test-bn.scm
@@ -6,6 +6,11 @@
    " object bn1 = Cyc_int2bignum2(data, obj_obj2int(fx1));
      object bn2 = Cyc_int2bignum2(data, obj_obj2int(fx2));
      object result = bignum2_plus_unsigned(data, bn1, bn2, 0); // TODO: int negp);
+if(is_value_type(result)) {
+  printf(\"fixnum result\\n\");
+} else if (type_of(result) == bignum2_tag) {
+  printf(\"bignum result\\n\");
+}
      return_closcall1(data, k, result); 
      ")
  (define-c test-bn
@@ -32,10 +37,11 @@
     (write row)
     (newline))
   (list
+    (test-plus #x0FFFffff #x0FFFffff)
+    (test-plus #x2FFFffff #x2FFFffff)
     (test-plus 1 2)
     ;; TODO: (fixnum? (test-plus 1 2))
     (test-plus -1 2)
-    (test-plus #x0FFFffff #x0FFFffff)
     (test-bn 123456789 )
     (test-bn 123456789 )
     (test-larger-bn 0 #x0FFF0001 )