diff --git a/bignum.c b/bignum.c
index fd0c2b71..7e3235c7 100644
--- a/bignum.c
+++ b/bignum.c
@@ -29,7 +29,7 @@ sexp sexp_make_bignum (sexp ctx, sexp_uint_t len) {
 sexp sexp_fixnum_to_bignum (sexp ctx, sexp a) {
   sexp res = sexp_make_bignum(ctx, 1);
   if (!sexp_exceptionp(res)) {
-    sexp_bignum_data(res)[0] = sexp_unbox_fixnum(sexp_fx_abs(a));
+    sexp_bignum_data(res)[0] = sexp_unbox_fx_abs(a);
     sexp_bignum_sign(res) = sexp_fx_sign(a);
   }
   return res;
@@ -326,9 +326,9 @@ sexp sexp_bignum_add_fixnum (sexp ctx, sexp a, sexp b) {
   sexp_gc_preserve1(ctx, c);
   c = sexp_copy_bignum(ctx, NULL, a, 0);
   if (sexp_bignum_sign(c) == sexp_fx_sign(b))
-    c = sexp_bignum_fxadd(ctx, c, sexp_unbox_fixnum(sexp_fx_abs(b)));
+    c = sexp_bignum_fxadd(ctx, c, sexp_unbox_fx_abs(b));
   else
-    c = sexp_bignum_fxsub(ctx, c, sexp_unbox_fixnum(sexp_fx_abs(b)));
+    c = sexp_bignum_fxsub(ctx, c, sexp_unbox_fx_abs(b));
   sexp_gc_release1(ctx);
   return c;
 }
@@ -367,7 +367,7 @@ sexp sexp_bignum_sub_digits (sexp ctx, sexp dst, sexp a, sexp b) {
 
 sexp sexp_bignum_add_digits (sexp ctx, sexp dst, sexp a, sexp b) {
   sexp_uint_t alen=sexp_bignum_hi(a), blen=sexp_bignum_hi(b),
-    carry=0, i, n, *adata, *bdata, *cdata;
+    carry=0, i, old_a, p_sum, *adata, *bdata, *cdata;
   sexp_gc_var1(c);
   if (alen < blen) return sexp_bignum_add_digits(ctx, dst, b, a);
   sexp_gc_preserve1(ctx, c);
@@ -377,9 +377,11 @@ sexp sexp_bignum_add_digits (sexp ctx, sexp dst, sexp a, sexp b) {
   bdata = sexp_bignum_data(b);
   cdata = sexp_bignum_data(c);
   for (i=0; i<blen; i++) {
-    n = adata[i];
-    cdata[i] = n + bdata[i] + carry;
-    carry = (n > (SEXP_UINT_T_MAX - bdata[i] - carry) ? 1 : 0);
+    old_a = adata[i]; /* adata may alias cdata */
+    p_sum = adata[i] + bdata[i];
+    cdata[i] = p_sum + carry;
+    carry = (old_a > (SEXP_UINT_T_MAX - bdata[i]) ? 1 : 0)
+          + (p_sum > (SEXP_UINT_T_MAX - carry) ? 1 : 0);
   }
   for ( ; carry && (i<alen); i++) {
     carry = (cdata[i] == SEXP_UINT_T_MAX ? 1 : 0);
@@ -599,7 +601,7 @@ sexp sexp_bignum_remainder (sexp ctx, sexp a, sexp b) {
 }
 
 sexp sexp_bignum_expt (sexp ctx, sexp a, sexp b) {
-  sexp_sint_t e = sexp_unbox_fixnum(sexp_fx_abs(b));
+  sexp_sint_t e = sexp_unbox_fx_abs(b);
   sexp_gc_var2(res, acc);
   sexp_gc_preserve2(ctx, res, acc);
   res = sexp_fixnum_to_bignum(ctx, SEXP_ONE);
@@ -1390,7 +1392,7 @@ sexp sexp_mul (sexp ctx, sexp a, sexp b) {
     r = (a==SEXP_ZERO ? a : sexp_make_flonum(ctx, sexp_fixnum_to_double(a)*sexp_flonum_value(b)));
     break;
   case SEXP_NUM_FIX_BIG:
-    r = sexp_bignum_fxmul(ctx, NULL, b, sexp_unbox_fixnum(sexp_fx_abs(a)), 0);
+    r = sexp_bignum_fxmul(ctx, NULL, b, sexp_unbox_fx_abs(a), 0);
     sexp_bignum_sign(r) = sexp_fx_sign(a) * sexp_bignum_sign(b);
     r = sexp_bignum_normalize(r);
     break;
@@ -1473,7 +1475,7 @@ sexp sexp_div (sexp ctx, sexp a, sexp b) {
     tmp = sexp_make_ratio(ctx, a, b);
     r = sexp_ratio_normalize(ctx, tmp, SEXP_FALSE);
 #else
-    r = sexp_make_flonum(ctx, sexp_fixnum_to_double(a)/sexp_bignum_to_double(b)); 
+    r = sexp_make_flonum(ctx, sexp_fixnum_to_double(a)/sexp_bignum_to_double(b));
 #endif
     break;
   case SEXP_NUM_FLO_FIX:
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 119a3a34..f21ac4f9 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -1182,6 +1182,8 @@ SEXP_API sexp sexp_symbol_table[SEXP_SYMBOL_TABLE_SIZE];
 #define sexp_fx_neg(a)    (sexp_make_fixnum(-(sexp_unbox_fixnum(a))))
 #define sexp_fx_abs(a)    ((((sexp_sint_t)a) < 0) ? sexp_fx_neg(a) : a)
 
+#define sexp_unbox_fx_abs(a) ((((sexp_sint_t)a) < 0) ? -sexp_unbox_fixnum(a) : sexp_unbox_fixnum(a))
+
 #define sexp_fp_add(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) + sexp_flonum_value(b)))
 #define sexp_fp_sub(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) - sexp_flonum_value(b)))
 #define sexp_fp_mul(x,a,b) (sexp_make_flonum(x, sexp_flonum_value(a) * sexp_flonum_value(b)))
diff --git a/tests/numeric-tests.scm b/tests/numeric-tests.scm
index 6fc7c06e..f755c79c 100644
--- a/tests/numeric-tests.scm
+++ b/tests/numeric-tests.scm
@@ -143,6 +143,79 @@
          -9223372036854775808))
     (sign-combinations M7 (+ 1 (expt 2 64))))
 
+;; fixnum-bignum boundaries (machine word - 1 bit for sign - 2 bits for tag)
+
+(test 8191 (- -8191))
+(test 8192 (- -8192))
+(test 8193 (- -8193))
+
+(test 536870911 (- -536870911))
+(test 536870912 (- -536870912))
+(test 536870913 (- -536870913))
+
+(test 2305843009213693951 (- -2305843009213693951))
+(test 2305843009213693952 (- -2305843009213693952))
+(test 2305843009213693953 (- -2305843009213693953))
+
+(test 42535295865117307932921825928971026431 (- -42535295865117307932921825928971026431))
+(test 42535295865117307932921825928971026432 (- -42535295865117307932921825928971026432))
+(test 42535295865117307932921825928971026433 (- -42535295865117307932921825928971026433))
+
+(test '((536879104 -536862720 4398046511104 0 8192)
+        (536862720 -536879104 -4398046511104 0 -8192)
+        (-536862720 536879104 -4398046511104 0 8192)
+        (-536879104 536862720 4398046511104 0 -8192))
+    (sign-combinations (expt 2 13) (expt 2 29)))
+
+(test '((536879104 536862720 4398046511104 65536 0)
+        (-536862720 -536879104 -4398046511104 -65536 0)
+        (536862720 536879104 -4398046511104 -65536 0)
+        (-536879104 -536862720 4398046511104 65536 0))
+    (sign-combinations (expt 2 29) (expt 2 13)))
+
+(test '((2305843009750564864 -2305843008676823040 1237940039285380274899124224 0 536870912)
+        (2305843008676823040 -2305843009750564864 -1237940039285380274899124224 0 -536870912)
+        (-2305843008676823040 2305843009750564864 -1237940039285380274899124224 0 536870912)
+        (-2305843009750564864 2305843008676823040 1237940039285380274899124224 0 -536870912))
+    (sign-combinations (expt 2 29) (expt 2 61)))
+
+(test '((2305843009750564864 2305843008676823040 1237940039285380274899124224 4294967296 0)
+        (-2305843008676823040 -2305843009750564864 -1237940039285380274899124224 -4294967296 0)
+        (2305843008676823040 2305843009750564864 -1237940039285380274899124224 -4294967296 0)
+        (-2305843009750564864 -2305843008676823040 1237940039285380274899124224 4294967296 0))
+    (sign-combinations (expt 2 61) (expt 2 29)))
+
+(test '((42535295865117307935227668938184720384 -42535295865117307930615982919757332480
+         98079714615416886934934209737619787751599303819750539264 0 2305843009213693952)
+        (42535295865117307930615982919757332480 -42535295865117307935227668938184720384
+         -98079714615416886934934209737619787751599303819750539264 0 -2305843009213693952)
+        (-42535295865117307930615982919757332480 42535295865117307935227668938184720384
+         -98079714615416886934934209737619787751599303819750539264 0 2305843009213693952)
+        (-42535295865117307935227668938184720384 42535295865117307930615982919757332480
+         98079714615416886934934209737619787751599303819750539264 0 -2305843009213693952))
+    (sign-combinations (expt 2 61) (expt 2 125)))
+
+(test '((42535295865117307935227668938184720384 42535295865117307930615982919757332480
+         98079714615416886934934209737619787751599303819750539264 18446744073709551616 0)
+        (-42535295865117307930615982919757332480 -42535295865117307935227668938184720384
+         -98079714615416886934934209737619787751599303819750539264 -18446744073709551616 0)
+        (42535295865117307930615982919757332480 42535295865117307935227668938184720384
+         -98079714615416886934934209737619787751599303819750539264 -18446744073709551616 0)
+        (-42535295865117307935227668938184720384 -42535295865117307930615982919757332480
+         98079714615416886934934209737619787751599303819750539264 18446744073709551616 0))
+    (sign-combinations (expt 2 125) (expt 2 61)))
+
+;; Regression tests for an overflow in bignum addition
+(test 8589869056
+    (+ 4294934528 4294934528))
+(test 36893488143124135936
+    (+ 18446744071562067968 18446744071562067968))
+(test 680564733841876926908302470789826871296
+    (+ 340282366920938463454151235394913435648 340282366920938463454151235394913435648))
+(test 231584178474632390847141970017375815706199686964360189615451793408394491068416
+    (+ 115792089237316195423570985008687907853099843482180094807725896704197245534208
+       115792089237316195423570985008687907853099843482180094807725896704197245534208))
+
 (test #f (< +nan.0 +nan.0))
 (test #f (<= +nan.0 +nan.0))
 (test #f (= +nan.0 +nan.0))