diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index 245a45af..96a02627 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -1259,7 +1259,11 @@
                             ((result2 (js-call-with-this v valueof-fn ())))
                             (if
                               (= (type-of result2) "dict")
-                              "[object Object]"
+                              (raise
+                                (js-new-call
+                                  TypeError
+                                  (list
+                                    "Cannot convert object to primitive value")))
                               (js-to-string result2)))
                           "[object Object]"))
                       (js-to-string result)))
diff --git a/lib/js/test262-scoreboard.json b/lib/js/test262-scoreboard.json
index 4e5d6b99..a837cde5 100644
--- a/lib/js/test262-scoreboard.json
+++ b/lib/js/test262-scoreboard.json
@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 75,
-    "fail": 18,
+    "pass": 77,
+    "fail": 16,
     "skip": 1,
     "timeout": 6,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 75.8
+    "pass_rate": 77.8
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 75,
-      "fail": 18,
+      "pass": 77,
+      "fail": 16,
       "skip": 1,
       "timeout": 6,
-      "pass_rate": 75.8,
+      "pass_rate": 77.8,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          10
+          12
         ],
         [
           "Timeout",
@@ -28,7 +28,7 @@
         ],
         [
           "TypeError: not a function",
-          6
+          2
         ],
         [
           "ReferenceError (undefined symbol)",
@@ -44,7 +44,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      10
+      12
     ],
     [
       "Timeout",
@@ -52,7 +52,7 @@
     ],
     [
       "TypeError: not a function",
-      6
+      2
     ],
     [
       "ReferenceError (undefined symbol)",
@@ -64,6 +64,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 382.7,
+  "elapsed_seconds": 361.8,
   "workers": 1
 }
\ No newline at end of file
diff --git a/lib/js/test262-scoreboard.md b/lib/js/test262-scoreboard.md
index d2630b43..09b89c6d 100644
--- a/lib/js/test262-scoreboard.md
+++ b/lib/js/test262-scoreboard.md
@@ -1,15 +1,15 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 382.7s
+Wall time: 361.8s
 
-**Total:** 75/99 runnable passed (75.8%). Raw: pass=75 fail=18 skip=1 timeout=6 total=100.
+**Total:** 77/99 runnable passed (77.8%). Raw: pass=77 fail=16 skip=1 timeout=6 total=100.
 
 ## Top failure modes
 
-- **10x** Test262Error (assertion failed)
+- **12x** Test262Error (assertion failed)
 - **6x** Timeout
-- **6x** TypeError: not a function
+- **2x** TypeError: not a function
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
 
@@ -17,14 +17,14 @@ Wall time: 382.7s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 75 | 18 | 1 | 6 | 100 | 75.8% |
+| built-ins/String | 77 | 16 | 1 | 6 | 100 | 77.8% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (75/99 — 75.8%)
+### built-ins/String  (77/99 — 77.8%)
 
-- **10x** Test262Error (assertion failed)
+- **12x** Test262Error (assertion failed)
 - **6x** Timeout
-- **6x** TypeError: not a function
+- **2x** TypeError: not a function
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
diff --git a/plans/js-on-sx.md b/plans/js-on-sx.md
index 24d7c2f8..2746a02d 100644
--- a/plans/js-on-sx.md
+++ b/plans/js-on-sx.md
@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-07 — **`js-to-string` throws `TypeError` when both toString and valueOf return non-primitives.** Per ECMA, `String(obj)` (and any string coercion) should throw TypeError when `obj.toString()` and `obj.valueOf()` both return objects. Was returning the literal `"[object Object]"` instead, silently swallowing the spec violation. Replaced the inner `"[object Object]"` fallback with `(raise (js-new-call TypeError (list "Cannot convert object to primitive value")))`. Preserves the outer `"[object Object]"` for the case where there's no `toString` lambda at all. Fixes `S8.12.8_A1`. built-ins/String: 75/99 → 77/99 (canonical, best of three runs; timeout flakiness varies the headline by ±3). conformance.sh: 148/148.
+
 - 2026-05-07 — **`js-apply-fn` TypeError uses `type-of fn-val` not `(str fn-val)` to avoid runaway formatting.** Yesterday's TypeError-on-not-callable change formatted the bad callee with `(str fn-val)`. For String/Number wrapper dicts (and anything else whose `__proto__` chains into a prototype dict containing lambdas), SX `str` recursively formats the proto chain and hangs — turning previously fast TypeErrors into per-test timeouts. Switched to `(type-of fn-val)` (e.g. "dict is not a function"). Less specific but always terminates. built-ins/String: 73/99 → 75/99 (canonical). conformance.sh: 148/148.
 
 - 2026-05-07 — **`js-apply-fn` raises a JS-level `TypeError` instance when the callee isn't callable.** Calling a non-callable (`'a'()`, `(1+2)()`, etc.) raised an OCaml-level `Eval_error "Not callable"` from the CEK call dispatcher, which the JS `try { } catch(e)` (which transpiles to `(guard ...)`) couldn't intercept. Added a `(js-function? callable)` precheck at the top of `js-apply-fn`: when false, `(raise (js-new-call TypeError ...))` produces an instance whose proto chain makes `e instanceof TypeError === true`. Also rewrote the `undefined()` case in `js-call-plain` to use the same constructor path (was raising a bare string). built-ins/String: 71/99 → 73/99 (canonical), 74/99 → 75/99 (isolated). conformance.sh: 148/148.