diff --git a/ext/rapidjson/encoder.hh b/ext/rapidjson/encoder.hh
index 53cd592..319bdb2 100644
--- a/ext/rapidjson/encoder.hh
+++ b/ext/rapidjson/encoder.hh
@@ -60,19 +60,14 @@ class RubyObjectEncoder {
     }
 
     void encode_bignum(VALUE b) {
-        b = rb_big_norm(b);
-        if (FIXNUM_P(b)) {
-            return encode_fixnum(b);
-        }
-
-        bool negative = rb_big_cmp(b, INT2FIX(0)) == INT2FIX(-1);
-        if (negative) {
-            long long ll = rb_big2ll(b);
-            writer.Int64(ll);
-        } else {
-            unsigned long long ull = rb_big2ull(b);
-            writer.Uint64(ull);
-        }
+        // Some T_BIGNUM might be small enough to fit in long long or unsigned long long
+        // but this being the slow path, it's not really worth it.
+        VALUE str = rb_funcall(b, id_to_s, 0);
+        Check_Type(str, T_STRING);
+
+        // We should be able to use RawNumber here, but it's buggy
+        // https://github.com/Tencent/rapidjson/issues/852
+        writer.RawValue(RSTRING_PTR(str), RSTRING_LEN(str), kNumberType);
     }
 
     void encode_float(VALUE v) {
diff --git a/test/test_encoder.rb b/test/test_encoder.rb
index 60cfe6a..75b8a8f 100644
--- a/test/test_encoder.rb
+++ b/test/test_encoder.rb
@@ -33,6 +33,10 @@ def test_encode_bignum
     assert_equal "18446744073709551615", encode(2**64 - 1)
   end
 
+  def test_encore_arbitrary_size_num
+    assert_equal "340282366920938463463374607431768211456", encode(2**128)
+  end
+
   def test_encode_fixnum_exponents
     tests = []
     0.upto(65) do |exponent|