diff --git a/src/modules/url.c b/src/modules/url.c
index b00f4c1..98678eb 100644
--- a/src/modules/url.c
+++ b/src/modules/url.c
@@ -47,7 +47,14 @@ static ant_value_t usp_array_len(ant_t *js, ant_value_t arr, ant_offset_t *out)
   ant_value_t length = js_get(js, arr, "length");
   if (is_err(length)) return length;
   if (js->thrown_exists) return mkval(T_ERR, 0);
-  if (vtype(length) == T_NUM) *out = (ant_offset_t)tod(length);
+  
+  double len_num = js_to_number(js, length);
+  if (js->thrown_exists) return mkval(T_ERR, 0);
+  
+  if (isnan(len_num) || !isfinite(len_num))
+    return js_mkerr_typed(js, JS_ERR_TYPE, "URLSearchParams sequence length must be finite");
+  
+  *out = (ant_offset_t)js_to_uint32(len_num);
   return js_mkundef();
 }
 
diff --git a/tests/test_proxy_array_flat_urlsearchparams.cjs b/tests/test_proxy_array_flat_urlsearchparams.cjs
index fe3f26e..dbd8d22 100644
--- a/tests/test_proxy_array_flat_urlsearchparams.cjs
+++ b/tests/test_proxy_array_flat_urlsearchparams.cjs
@@ -31,6 +31,14 @@ assert(params.toString() === "a=b", "URLSearchParams should accept proxied array
 assert(sawOuterLength, "URLSearchParams should observe outer proxy length");
 assert(sawPairLength, "URLSearchParams should observe pair proxy length");
 
+const stringLengthParams = new URLSearchParams(new Proxy([pair], {
+  get(target, key, receiver) {
+    if (key === "length") return "1";
+    return Reflect.get(target, key, receiver);
+  },
+}));
+assert(stringLengthParams.toString() === "a=b", "URLSearchParams should convert proxy length with ToNumber");
+
 assertThrows(
   function () {
     new URLSearchParams(new Proxy([pair], {
@@ -70,6 +78,32 @@ assertThrows(
   "URLSearchParams should propagate pair proxy value errors"
 );
 
+assertThrows(
+  function () {
+    new URLSearchParams(new Proxy([pair], {
+      get(target, key, receiver) {
+        if (key === "length") return NaN;
+        return Reflect.get(target, key, receiver);
+      },
+    }));
+  },
+  "finite",
+  "URLSearchParams should reject NaN proxy lengths"
+);
+
+assertThrows(
+  function () {
+    new URLSearchParams(new Proxy([pair], {
+      get(target, key, receiver) {
+        if (key === "length") return { valueOf() { throw new Error("length convert boom"); } };
+        return Reflect.get(target, key, receiver);
+      },
+    }));
+  },
+  "length convert boom",
+  "URLSearchParams should propagate proxy length conversion errors"
+);
+
 console.log("OK");
 
 function assertThrows(fn, expected, message) {