diff --git a/opt/bignum.c b/opt/bignum.c
index fb211725..d80fb6cb 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -434,8 +434,10 @@ sexp sexp_bignum_quotient (sexp ctx, sexp a, sexp b) {
 }
 
 sexp sexp_bignum_remainder (sexp ctx, sexp a, sexp b) {
-  sexp rem;
+  sexp_gc_var1(rem);
+  sexp_gc_preserve1(ctx, rem);
   sexp_bignum_quot_rem(ctx, &rem, a, b); /* discard quotient */
+  sexp_gc_release1(ctx);
   return rem;
 }
 
@@ -605,9 +607,9 @@ sexp sexp_mul (sexp ctx, sexp a, sexp b) {
 sexp sexp_div (sexp ctx, sexp a, sexp b) {
   int at=sexp_number_type(a), bt=sexp_number_type(b);
   double f;
-  sexp r=SEXP_VOID, rem;
-  sexp_gc_var1(tmp);
-  sexp_gc_preserve1(ctx, tmp);
+  sexp r=SEXP_VOID;
+  sexp_gc_var2(tmp, rem);
+  sexp_gc_preserve2(ctx, tmp, rem);
   switch ((at << 2) + bt) {
   case SEXP_NUM_NOT_NOT: case SEXP_NUM_NOT_FIX:
   case SEXP_NUM_NOT_FLO: case SEXP_NUM_NOT_BIG:
@@ -651,7 +653,7 @@ sexp sexp_div (sexp ctx, sexp a, sexp b) {
     r = sexp_make_flonum(ctx, sexp_bignum_to_double(a) / sexp_flonum_value(b));
     break;
   }
-  sexp_gc_release1(ctx);
+  sexp_gc_release2(ctx);
   return r;
 }
 
diff --git a/sexp.c b/sexp.c
index 2a3ff0f6..b6eab627 100644
--- a/sexp.c
+++ b/sexp.c
@@ -79,7 +79,7 @@ static struct sexp_struct _sexp_type_specs[] = {
   _DEF_TYPE(SEXP_STRING, 0, 0, 0, 0, 0, sexp_sizeof(string)+1, sexp_offsetof(string, length), 1, "string", NULL),
   _DEF_TYPE(SEXP_VECTOR, sexp_offsetof(vector, data), 0, 0, sexp_offsetof(vector, length), 1, sexp_sizeof(vector), sexp_offsetof(vector, length), sizeof(sexp), "vector", NULL),
   _DEF_TYPE(SEXP_FLONUM, 0, 0, 0, 0, 0, sexp_sizeof(flonum), 0, 0, "flonum", NULL),
-  _DEF_TYPE(SEXP_BIGNUM, 0, 0, 0, 0, 0, sexp_sizeof(bignum), sexp_offsetof(bignum, length), sizeof(sexp), "bignum", NULL),
+  _DEF_TYPE(SEXP_BIGNUM, 0, 0, 0, 0, 0, sexp_sizeof(bignum), sexp_offsetof(bignum, length), sizeof(sexp_uint_t), "bignum", NULL),
   _DEF_TYPE(SEXP_CPOINTER, sexp_offsetof(cpointer, parent), 1, 0, 0, 0, sexp_sizeof(cpointer), sexp_offsetof(cpointer, length), 1, "cpointer", NULL),
   _DEF_TYPE(SEXP_IPORT, sexp_offsetof(port, name), 2, 2, 0, 0, sexp_sizeof(port), 0, 0, "input-port", SEXP_FINALIZE_PORT),
   _DEF_TYPE(SEXP_OPORT, sexp_offsetof(port, name), 2, 2, 0, 0, sexp_sizeof(port), 0, 0, "output-port", SEXP_FINALIZE_PORT),