diff --git a/bignum.c b/bignum.c
index b20fb7c1..63859ddb 100644
--- a/bignum.c
+++ b/bignum.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);
diff --git a/tests/numeric-tests.scm b/tests/numeric-tests.scm
index 526810b5..f755c79c 100644
--- a/tests/numeric-tests.scm
+++ b/tests/numeric-tests.scm
@@ -205,6 +205,17 @@
          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))