diff --git a/c/builtins.c b/c/builtins.c
index 4001cdc..16453ca 100644
--- a/c/builtins.c
+++ b/c/builtins.c
@@ -748,7 +748,12 @@ static Value bi_string_ref(Value *a, int n, Env *e) {
     int idx = (int)as_number_int(a[1]);
     ULString *s = AS_STRING(a[0]);
     if (idx < 0 || idx >= (int)s->len) lisp_error("string-ref: index out of range");
-    return VAL_CHAR(s->data[idx]);
+    /* Cast through unsigned char so bytes 0x80..0xFF map to codepoints
+     * 128..255 rather than sign-extending to negative ints — emit-stream
+     * stores binary op bytes through (make-string 1 (integer->char b))
+     * + (string-ref s 0); without this, char->integer of 0xFF returned
+     * -1, so u64-le's serialization produced a sentinel-shaped giant. */
+    return VAL_CHAR((unsigned char)s->data[idx]);
 }
 
 static Value bi_substring(Value *a, int n, Env *e) {
@@ -1595,6 +1600,13 @@ static void pack_u64_slot(char *buf, Value v) {
     uint64_t u;
     if (v == VAL_FALSE) {
         u = 0xFFFFFFFFFFFFFFFFULL;
+    } else if (IS_BIGNUM(v)) {
+        /* Bignum sentinel (e.g. *no-slot* = 2^64-1) — extract low 64
+         * bits of magnitude. Python tier passes the literal big-int so
+         * cross-tier emit-stream code stays unchanged. */
+        Bignum *b = AS_BIGNUM(v);
+        u = b->n_limbs > 0 ? b->limbs[0] : 0;
+        if (b->sign < 0) u = ~u + 1;
     } else {
         u = (uint64_t)as_int(v);
     }