diff --git a/bridge/core/frame/window.cc b/bridge/core/frame/window.cc
index ab952f66c6..5a83e1fa18 100644
--- a/bridge/core/frame/window.cc
+++ b/bridge/core/frame/window.cc
@@ -34,26 +34,32 @@ AtomicString Window::btoa(const AtomicString& source, ExceptionState& exception_
     return AtomicString::Empty();
   size_t encode_len = modp_b64_encode_data_len(source.length());
   std::vector<char> buffer;
-  buffer.resize(encode_len);
+  buffer.resize(encode_len + 1);
+
+  std::string source_string = source.ToStdString(ctx());
+
+  const size_t output_size = modp_b64_encode(reinterpret_cast<char*>(buffer.data()),
+                                             source_string.c_str(), source.length());
+  const char* encode_str = buffer.data();
+  const size_t encode_str_len = strlen(encode_str);
 
-  const size_t output_size = modp_b64_encode_data(reinterpret_cast<char*>(buffer.data()),
-                                                  reinterpret_cast<const char*>(source.Character8()), source.length());
   assert(output_size == encode_len);
-  if (output_size != encode_len || buffer.empty()) {
+  if (output_size != encode_len || encode_str_len == 0) {
     exception_state.ThrowException(ctx(), ErrorType::TypeError, "The string encode failed.");
     return AtomicString::Empty();
   }
-
-  return {ctx(), buffer.data(), buffer.size()};
+  return {ctx(), encode_str, encode_str_len};
 }
 
 // Invokes modp_b64 without stripping whitespace.
-bool Base64DecodeRaw(const AtomicString& in, std::vector<uint8_t>& out, ModpDecodePolicy policy) {
+bool Base64DecodeRaw(JSContext* ctx, const AtomicString& in, std::vector<uint8_t>& out, ModpDecodePolicy policy) {
   size_t decode_len = modp_b64_decode_len(in.length());
   out.resize(decode_len);
 
+  std::string in_string = in.ToStdString(ctx);
+
   const size_t output_size = modp_b64_decode(reinterpret_cast<char*>(out.data()),
-                                             reinterpret_cast<const char*>(in.Character8()), in.length(), policy);
+                                             in_string.c_str(), in.length(), policy);
   if (output_size == MODP_B64_ERROR)
     return false;
   out.resize(output_size);
@@ -72,11 +78,11 @@ bool Base64Decode(JSContext* ctx, AtomicString in, std::vector<uint8_t>& out, Mo
       // TODO(csharrison): Most callers use String inputs so ToString() should
       // be fast. Still, we should add a RemoveCharacters method to StringView
       // to avoid a double allocation for non-String-backed StringViews.
-      return Base64DecodeRaw(in, out, policy) ||
-             Base64DecodeRaw(in.RemoveCharacters(ctx, &IsAsciiWhitespace), out, policy);
+      return Base64DecodeRaw(ctx, in, out, policy) ||
+             Base64DecodeRaw(ctx, in.RemoveCharacters(ctx, &IsAsciiWhitespace), out, policy);
     }
     case ModpDecodePolicy::kNoPaddingValidation: {
-      return Base64DecodeRaw(in, out, policy);
+      return Base64DecodeRaw(ctx, in, out, policy);
     }
     case ModpDecodePolicy::kStrict:
       return false;