diff --git a/io/binaryreader.cpp b/io/binaryreader.cpp
index e4d77bf..7d12006 100644
--- a/io/binaryreader.cpp
+++ b/io/binaryreader.cpp
@@ -100,7 +100,7 @@ istream::pos_type BinaryReader::readStreamsize()
  */
 string BinaryReader::readLengthPrefixedString()
 {
-    static const int maxPrefixLength = 4;
+    static constexpr int maxPrefixLength = 8;
     int prefixLength = 1;
     const byte beg = static_cast<byte>(m_stream->peek());
     byte mask = 0x80;
@@ -114,8 +114,7 @@ string BinaryReader::readLengthPrefixedString()
     memset(m_buffer, 0, maxPrefixLength);
     m_stream->read(m_buffer + (maxPrefixLength - prefixLength), prefixLength);
     *(m_buffer + (maxPrefixLength - prefixLength)) ^= mask;
-    uint32 prefix = BE::toUInt32(m_buffer);
-    return readString(prefix);
+    return readString(BE::toUInt64(m_buffer));
 }
 
 /*!
diff --git a/io/binarywriter.cpp b/io/binarywriter.cpp
index dd9e5b7..277c7d0 100644
--- a/io/binarywriter.cpp
+++ b/io/binarywriter.cpp
@@ -77,21 +77,18 @@ void BinaryWriter::setStream(ostream *stream, bool giveOwnership)
  */
 void BinaryWriter::writeLengthPrefixedString(const string &value)
 {
-    size_t length = value.length();
-    if (length < 0x80) {
-        m_buffer[0] = 0x80 | length;
-        m_stream->write(m_buffer, 1);
-    } else if (length < 0x4000) {
-        BE::getBytes(static_cast<uint16>(0x4000 | length), m_buffer);
-        m_stream->write(m_buffer, 2);
-    } else if (length < 0x200000) {
-        BE::getBytes(static_cast<uint32>(0x200000 | length), m_buffer);
-        m_stream->write(m_buffer + 1, 3);
-    } else if (length < 0x10000000) {
-        BE::getBytes(static_cast<uint32>(0x10000000 | length), m_buffer);
-        m_stream->write(m_buffer, 4);
-    } else {
+    const uint64 size = value.size();
+    uint64 boundCheck = 0x80;
+    byte prefixLength = 1;
+    for (; boundCheck != 0x8000000000000000; boundCheck <<= 7, ++prefixLength) {
+        if (size < boundCheck) {
+            BE::getBytes(size | boundCheck, m_buffer);
+            break;
+        }
+    }
+    if (prefixLength == 9) {
         throw ConversionException("The size of the string exceeds the maximum.");
     }
-    m_stream->write(value.c_str(), length);
+    m_stream->write(m_buffer + 8 - prefixLength, prefixLength);
+    m_stream->write(value.data(), static_cast<streamsize>(size));
 }
diff --git a/testfiles/some_data b/testfiles/some_data
index 3dfd5a7..0e85b07 100644
Binary files a/testfiles/some_data and b/testfiles/some_data differ
diff --git a/tests/iotests.cpp b/tests/iotests.cpp
index e0523d9..11f2a7e 100644
--- a/tests/iotests.cpp
+++ b/tests/iotests.cpp
@@ -1,5 +1,6 @@
 #include "./testutils.h"
 
+#include "../conversion/conversionexception.h"
 #include "../io/binaryreader.h"
 #include "../io/binarywriter.h"
 #include "../io/bitreader.h"
@@ -20,6 +21,7 @@ using namespace std;
 using namespace IoUtilities;
 using namespace TestUtilities;
 using namespace TestUtilities::Literals;
+using namespace ConversionUtilities;
 
 using namespace CPPUNIT_NS;
 
@@ -98,7 +100,7 @@ void IoTests::testBinaryReader()
     testFile.exceptions(ios_base::failbit | ios_base::badbit);
     testFile.open(TestUtilities::testFilePath("some_data"), ios_base::in | ios_base::binary);
     BinaryReader reader(&testFile);
-    CPPUNIT_ASSERT_EQUAL(reader.readStreamsize(), static_cast<istream::pos_type>(95));
+    CPPUNIT_ASSERT_EQUAL(reader.readStreamsize(), static_cast<istream::pos_type>(398));
     CPPUNIT_ASSERT(reader.readUInt16LE() == 0x0102u);
     CPPUNIT_ASSERT(reader.readUInt16BE() == 0x0102u);
     CPPUNIT_ASSERT(reader.readUInt24LE() == 0x010203u);
@@ -132,7 +134,14 @@ void IoTests::testBinaryReader()
     CPPUNIT_ASSERT(reader.readBool() == true);
     CPPUNIT_ASSERT(reader.readString(3) == "abc");
     CPPUNIT_ASSERT(reader.readLengthPrefixedString() == "ABC");
+    CPPUNIT_ASSERT(reader.readLengthPrefixedString() == "01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901"
+                                                        "23456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123"
+                                                        "45678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345"
+                                                        "678901234567890123456789");
     CPPUNIT_ASSERT(reader.readTerminatedString() == "def");
+    CPPUNIT_ASSERT_THROW(reader.readLengthPrefixedString(), ConversionException);
+    CPPUNIT_ASSERT_MESSAGE("pos in stream not advanced on conversion error", reader.readByte() == 0);
+
     // test ownership
     reader.setStream(nullptr, true);
     reader.setStream(new fstream(), true);
@@ -156,7 +165,7 @@ void IoTests::testBinaryWriter()
     // prepare output stream
     stringstream outputStream(ios_base::in | ios_base::out | ios_base::binary);
     outputStream.exceptions(ios_base::failbit | ios_base::badbit);
-    char testData[95];
+    char testData[397];
     outputStream.rdbuf()->pubsetbuf(testData, sizeof(testData));
 
     // write test data
@@ -205,6 +214,9 @@ void IoTests::testBinaryWriter()
     writer.writeBool(true);
     writer.writeString("abc");
     writer.writeLengthPrefixedString("ABC");
+    writer.writeLengthPrefixedString("012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890"
+                                     "123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901"
+                                     "234567890123456789012345678901234567890123456789012345678901234567890123456789");
     writer.writeTerminatedString("def");
 
     // test written values