diff --git a/AK/String.cpp b/AK/String.cpp
index e430dede5ea..9c3ae084eeb 100644
--- a/AK/String.cpp
+++ b/AK/String.cpp
@@ -61,19 +61,30 @@ ErrorOr<String> String::from_utf8(StringView view)
     return result;
 }
 
+ErrorOr<String> String::from_utf16(Utf16View const& utf16)
+{
+    return utf16.to_utf8();
+}
+
 ErrorOr<String> String::from_utf16_le(ReadonlyBytes bytes)
 {
     if (!validate_utf16_le(bytes))
         return Error::from_string_literal("String::from_utf16_le: Input was not valid UTF-16LE");
     if (bytes.is_empty())
         return String {};
-    char16_t const* utf16_data = reinterpret_cast<char16_t const*>(bytes.data());
-    size_t utf16_length = bytes.size() / 2;
-    size_t max_utf8_length = simdutf::utf8_length_from_utf16(utf16_data, utf16_length);
-    Vector<u8> buffer;
-    buffer.resize(max_utf8_length);
-    auto utf8_length = simdutf::convert_utf16le_to_utf8(utf16_data, utf16_length, reinterpret_cast<char*>(buffer.data()));
-    return String::from_utf8_without_validation(ReadonlyBytes { buffer.data(), utf8_length });
+
+    auto const* utf16_data = reinterpret_cast<char16_t const*>(bytes.data());
+    auto utf16_length = bytes.size() / 2;
+    auto utf8_length = simdutf::utf8_length_from_utf16le(utf16_data, utf16_length);
+
+    String result;
+    TRY(result.replace_with_new_string(utf8_length, [&](Bytes buffer) -> ErrorOr<void> {
+        [[maybe_unused]] auto result = simdutf::convert_utf16le_to_utf8(utf16_data, utf16_length, reinterpret_cast<char*>(buffer.data()));
+        ASSERT(result == buffer.size());
+        return {};
+    }));
+
+    return result;
 }
 
 ErrorOr<String> String::from_utf16_be(ReadonlyBytes bytes)
@@ -82,18 +93,19 @@ ErrorOr<String> String::from_utf16_be(ReadonlyBytes bytes)
         return Error::from_string_literal("String::from_utf16_be: Input was not valid UTF-16BE");
     if (bytes.is_empty())
         return String {};
-    char16_t const* utf16_data = reinterpret_cast<char16_t const*>(bytes.data());
-    size_t utf16_length = bytes.size() / 2;
-    size_t max_utf8_length = simdutf::utf8_length_from_utf16(utf16_data, utf16_length);
-    Vector<u8> buffer;
-    buffer.resize(max_utf8_length);
-    auto utf8_length = simdutf::convert_utf16be_to_utf8(utf16_data, utf16_length, reinterpret_cast<char*>(buffer.data()));
-    return String::from_utf8_without_validation(ReadonlyBytes { buffer.data(), utf8_length });
-}
 
-ErrorOr<String> String::from_utf16(Utf16View const& utf16)
-{
-    return utf16.to_utf8();
+    auto const* utf16_data = reinterpret_cast<char16_t const*>(bytes.data());
+    auto utf16_length = bytes.size() / 2;
+    auto utf8_length = simdutf::utf8_length_from_utf16be(utf16_data, utf16_length);
+
+    String result;
+    TRY(result.replace_with_new_string(utf8_length, [&](Bytes buffer) -> ErrorOr<void> {
+        [[maybe_unused]] auto result = simdutf::convert_utf16be_to_utf8(utf16_data, utf16_length, reinterpret_cast<char*>(buffer.data()));
+        ASSERT(result == buffer.size());
+        return {};
+    }));
+
+    return result;
 }
 
 ErrorOr<String> String::from_stream(Stream& stream, size_t byte_count)