diff --git a/CUETools.Codecs/BitWriter.cs b/CUETools.Codecs/BitWriter.cs
index 707c796..71ed7d4 100644
--- a/CUETools.Codecs/BitWriter.cs
+++ b/CUETools.Codecs/BitWriter.cs
@@ -139,7 +139,7 @@ namespace CUETools.Codecs
             //assert(bits == 32 || val < (1U << bits));
 
             if (bits == 0 || eof) return;
-            if (bits <= bit_left_m)
+            if (bits < bit_left_m)
             {
                 bit_left_m -= bits;
                 bit_buf_m |= val << bit_left_m;
@@ -174,8 +174,10 @@ namespace CUETools.Codecs
                     buffer[buf_ptr_m + 0] = (byte)(bb & 0xFF);
                     buf_ptr_m += 8;
                 }
-                bit_left_m = 64 + bit_left_m - bits;
-                bit_buf_m = val << bit_left_m;
+                // cannot do this in one shift, because bit_left_m can be 64,
+                // 
+                bit_left_m += 64 - bits;
+                bit_buf_m = bit_left_m == 64 ? 0 : val << bit_left_m;
             }
         }
 
@@ -284,7 +286,7 @@ namespace CUETools.Codecs
                 // write quotient in unary
                 int q = (v >> k) + 1;
                 int bits = k + q;
-                while (bits > 56)
+                while (bits > 64)
                 {
 #if DEBUG
                     if (buf + 1 > fixedbuf + buf_end)
@@ -302,7 +304,7 @@ namespace CUETools.Codecs
                 // write remainder in binary using 'k' bits
                 //writebits_fast(k + q, (uint)((v & ((1 << k) - 1)) | (1 << k)), ref buf);
                 ulong val = (uint)((v & ((1 << k) - 1)) | (1 << k));
-                if (bits <= bit_left)
+                if (bits < bit_left)
                 {
                     bit_left -= bits;
                     bit_buf |= val << bit_left;
@@ -335,8 +337,8 @@ namespace CUETools.Codecs
                     *(buf++) = (byte)(bb >> 16);
                     *(buf++) = (byte)(bb >> 8);
                     *(buf++) = (byte)(bb);
-                    bit_left = 64 + bit_left - bits;
-                    bit_buf = val << bit_left;
+                    bit_left += 64 - bits;
+                    bit_buf = (val << bit_left - 1) << 1;
                 }
             }
             crc16_m = crc16;
diff --git a/CUETools/CUETools.TestCodecs/BitWriterTest.cs b/CUETools/CUETools.TestCodecs/BitWriterTest.cs
new file mode 100644
index 0000000..13e5f29
--- /dev/null
+++ b/CUETools/CUETools.TestCodecs/BitWriterTest.cs
@@ -0,0 +1,106 @@
+using CUETools.Codecs;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+
+namespace CUETools.TestCodecs
+{
+    
+    
+    /// <summary>
+    ///This is a test class for BitWriterTest and is intended
+    ///to contain all BitWriterTest Unit Tests
+    ///</summary>
+    [TestClass()]
+    public class BitWriterTest
+    {
+
+
+        private TestContext testContextInstance;
+
+        /// <summary>
+        ///Gets or sets the test context which provides
+        ///information about and functionality for the current test run.
+        ///</summary>
+        public TestContext TestContext
+        {
+            get
+            {
+                return testContextInstance;
+            }
+            set
+            {
+                testContextInstance = value;
+            }
+        }
+
+        #region Additional test attributes
+        // 
+        //You can use the following additional attributes as you write your tests:
+        //
+        //Use ClassInitialize to run code before running the first test in the class
+        //[ClassInitialize()]
+        //public static void MyClassInitialize(TestContext testContext)
+        //{
+        //}
+        //
+        //Use ClassCleanup to run code after all tests in a class have run
+        //[ClassCleanup()]
+        //public static void MyClassCleanup()
+        //{
+        //}
+        //
+        //Use TestInitialize to run code before running each test
+        //[TestInitialize()]
+        //public void MyTestInitialize()
+        //{
+        //}
+        //
+        //Use TestCleanup to run code after each test has run
+        //[TestCleanup()]
+        //public void MyTestCleanup()
+        //{
+        //}
+        //
+        #endregion
+
+
+        /// <summary>
+        ///A test for writebits
+        ///</summary>
+        [TestMethod()]
+        public void writebitsTest()
+        {
+            byte[] buf = new byte[0x10000];
+            for (int seed = 0; seed < 10; seed++)
+            {
+                BitWriter target = new BitWriter(buf, 0, buf.Length);
+                var rnd = new Random(seed);
+                int count = 0;
+                do
+                {
+                    int bits = rnd.Next(0, 64) + 1;
+                    ulong val = (1U << (bits - 1));
+                    target.writebits(bits, val);
+                    count++;
+                }
+                while (target.Length < buf.Length - 32);
+                target.flush();
+                rnd = new Random(seed);
+                unsafe
+                {
+                    fixed (byte* ptr = buf)
+                    {
+                        BitReader reader = new BitReader(ptr, 0, buf.Length);
+                        for (int i = 0; i < count; i++)
+                        {
+                            int bits = rnd.Next(0, 64) + 1;
+                            ulong val = (1U << (bits - 1));
+                            ulong val1 = reader.readbits64(bits);
+                            Assert.AreEqual(val, val1, string.Format("i = {0}, bits = {1}, seed = {2}, pos = {3}", i, bits, seed, reader.Position));
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/CUETools/CUETools.TestCodecs/CUETools.TestCodecs.csproj b/CUETools/CUETools.TestCodecs/CUETools.TestCodecs.csproj
index 2955a6b..b0da770 100644
--- a/CUETools/CUETools.TestCodecs/CUETools.TestCodecs.csproj
+++ b/CUETools/CUETools.TestCodecs/CUETools.TestCodecs.csproj
@@ -66,6 +66,7 @@
   <ItemGroup>
     <Compile Include="AccurateRipVerifyTest.cs" />
     <Compile Include="ALACWriterTest.cs" />
+    <Compile Include="BitWriterTest.cs" />
     <Compile Include="CodecsTest.cs" />
     <Compile Include="Crc32Test.cs" />
     <Compile Include="FlacWriterTest.cs" />