diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 72fbe564..28f2fed8 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -474,10 +474,12 @@ sexp sexp_make_flonum(sexp ctx, double f);
 #endif
 
 #if SEXP_USE_BIGNUMS
-SEXP_API sexp sexp_make_integer(sexp ctx, sexp_sint_t x);
+SEXP_API sexp sexp_make_integer(sexp ctx, sexp_lsint_t x);
+SEXP_API sexp sexp_make_unsigned_integer(sexp ctx, sexp_luint_t x);
 #define sexp_exact_integerp(x) (sexp_fixnump(x) || sexp_bignump(x))
 #else
 #define sexp_make_integer(ctx, x) sexp_make_fixnum(x)
+#define sexp_make_unsigned_integer(ctx, x) sexp_make_fixnum(x)
 #define sexp_exact_integerp(x) sexp_fixnump(x)
 #endif
 
diff --git a/opt/bignum.c b/opt/bignum.c
index 90f71661..60215de8 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -25,14 +25,31 @@ sexp sexp_fixnum_to_bignum (sexp ctx, sexp a) {
   return res;
 }
 
-sexp sexp_make_integer (sexp ctx, sexp_sint_t x) {
+sexp sexp_make_integer (sexp ctx, sexp_lsint_t x) {
   sexp res;
   if ((SEXP_MIN_FIXNUM < x) && (x < SEXP_MAX_FIXNUM)) {
     res = sexp_make_fixnum(x);
   } else {
     res = sexp_make_bignum(ctx, 1);
-    sexp_bignum_sign(res) = (x < 0 ? -1 : 1);
-    sexp_bignum_data(res)[0] = x * sexp_bignum_sign(res);
+    if (x < 0) {
+      sexp_bignum_sign(res) = -1;
+      sexp_bignum_data(res)[0] = -x;
+    } else {
+      sexp_bignum_sign(res) = 1;
+      sexp_bignum_data(res)[0] = x;
+    }
+  }
+  return res;
+}
+
+sexp sexp_make_unsigned_integer (sexp ctx, sexp_luint_t x) {
+  sexp res;
+  if (x < SEXP_MAX_FIXNUM) {
+    res = sexp_make_fixnum(x);
+  } else {
+    res = sexp_make_bignum(ctx, 1);
+    sexp_bignum_sign(res) = 1;
+    sexp_bignum_data(res)[0] = x;
   }
   return res;
 }
diff --git a/tools/genstubs.scm b/tools/genstubs.scm
index 037e853e..4f248554 100755
--- a/tools/genstubs.scm
+++ b/tools/genstubs.scm
@@ -452,7 +452,9 @@
       (cat "sexp_make_boolean(" val ")"))
      ((eq? base 'time_t)
       (cat "sexp_make_integer(ctx, sexp_shift_epoch(" val "))"))
-     ((int-type? base)
+     ((unsigned-int-type? base)
+      (cat "sexp_make_unsigned_integer(ctx, " val ")"))
+     ((signed-int-type? base)
       (cat "sexp_make_integer(ctx, " val ")"))
      ((eq? base 'char)
       (if (type-array type)