diff --git a/src/main/java/net/openhft/chronicle/map/AbstractChronicleMapBuilder.java b/src/main/java/net/openhft/chronicle/map/AbstractChronicleMapBuilder.java
index b464fb929..a09c46587 100644
--- a/src/main/java/net/openhft/chronicle/map/AbstractChronicleMapBuilder.java
+++ b/src/main/java/net/openhft/chronicle/map/AbstractChronicleMapBuilder.java
@@ -138,7 +138,7 @@ public abstract class AbstractChronicleMapBuilder<K, V,
             MapEventListeners.nop();
     private V defaultValue = null;
     private DefaultValueProvider<K, V> defaultValueProvider = NullValueProvider.INSTANCE;
-    private PrepareValueBytes<K> prepareValueBytes = null;
+    private PrepareValueBytes<K, V> prepareValueBytes = null;
     private byte identifier = -1;
 
     private StatelessBuilder statelessBuilder;
@@ -735,7 +735,7 @@ public abstract class AbstractChronicleMapBuilder<K, V,
         return self();
     }
 
-    B prepareValueBytesOnAcquire(@NotNull PrepareValueBytes<K> prepareValueBytes) {
+    B prepareValueBytesOnAcquire(@NotNull PrepareValueBytes<K, V> prepareValueBytes) {
         this.prepareValueBytes = prepareValueBytes;
         this.defaultValue = null;
         this.defaultValueProvider = null;
diff --git a/src/main/java/net/openhft/chronicle/map/OffHeapUpdatableChronicleMapBuilder.java b/src/main/java/net/openhft/chronicle/map/OffHeapUpdatableChronicleMapBuilder.java
index 3d60a400c..4a8aba2df 100644
--- a/src/main/java/net/openhft/chronicle/map/OffHeapUpdatableChronicleMapBuilder.java
+++ b/src/main/java/net/openhft/chronicle/map/OffHeapUpdatableChronicleMapBuilder.java
@@ -38,7 +38,7 @@ public final class OffHeapUpdatableChronicleMapBuilder<K, V>
 
     OffHeapUpdatableChronicleMapBuilder(Class<K> keyClass, Class<V> valueClass) {
         super(keyClass, valueClass);
-        prepareValueBytesOnAcquire(new ZeroOutValueBytes<K>(valueSize()));
+        prepareValueBytesOnAcquire(new ZeroOutValueBytes<K, V>(valueSize()));
     }
 
     @Override
@@ -118,19 +118,21 @@ public final class OffHeapUpdatableChronicleMapBuilder<K, V>
     /**
      * Configures the procedure which is called on the bytes, which later the returned value is
      * pointing to, if the key is absent, on {@link ChronicleMap#acquireUsing(Object, Object)
-     * acquireUsing()} call on maps, created by this builder.
+     * acquireUsing()} call on maps, created by this builder. See {@link PrepareValueBytes} for
+     * more information.
      *
      * <p>The default preparation callback zeroes out the value bytes.
      *
      * @param prepareValueBytes what to do with the value bytes before assigning them into the
      *                          {@link Byteable} value to return from {@code acquireUsing()} call
      * @return this builder back
+     * @see PrepareValueBytes
      * @see #defaultValue(Object)
      * @see #defaultValueProvider(DefaultValueProvider)
      */
     @Override
     public OffHeapUpdatableChronicleMapBuilder<K, V> prepareValueBytesOnAcquire(
-            @NotNull PrepareValueBytes<K> prepareValueBytes) {
+            @NotNull PrepareValueBytes<K, V> prepareValueBytes) {
         return super.prepareValueBytesOnAcquire(prepareValueBytes);
     }
 }
diff --git a/src/main/java/net/openhft/chronicle/map/PrepareValueBytes.java b/src/main/java/net/openhft/chronicle/map/PrepareValueBytes.java
index bcaca2470..e2b726fec 100644
--- a/src/main/java/net/openhft/chronicle/map/PrepareValueBytes.java
+++ b/src/main/java/net/openhft/chronicle/map/PrepareValueBytes.java
@@ -17,10 +17,46 @@
 package net.openhft.chronicle.map;
 
 import net.openhft.lang.io.Bytes;
+import net.openhft.lang.model.Byteable;
 
 import java.io.Serializable;
 
-public interface PrepareValueBytes<K> extends Serializable {
+/**
+ * Strategy of preparing bytes of off-heap memory before assigning them into {@link Byteable}
+ * value during {@link ChronicleMap#acquireUsing(Object, Object)
+ * chronicleMap.acquireUsing()} call, when the given key is absent in the map. Example: if you
+ * want {@code 1L} to be the off-heap updatable ChronicleMap's {@linkplain
+ * OffHeapUpdatableChronicleMapBuilder#defaultValue(Object) default value}, the most simple way
+ * to achieve this is to {@linkplain OffHeapUpdatableChronicleMapBuilder#prepareValueBytesOnAcquire(
+ * PrepareValueBytes) configure} custom {@code PrepareValueBytes} strategy in {@link
+ * OffHeapUpdatableChronicleMapBuilder}: <pre>{@code
+ * OffHeapUpdatableChronicleMapBuilder.of(Key.class, LongValue.class)
+ *     .prepareValueBytesOnAcquire((bytes, k) -> bytes.writeLong(1L))
+ *     ...
+ *     .create();}</pre>
+ *
+ * <p>Although the value type is not used in the API, {@link PrepareValueBytes} is not intended
+ * to be generic by value type, because strategy should know the value layout and
+ * {@linkplain Byteable#maxSize() how much bytes} does it take in memory.
+ *
+ * @param <K> the key type of {@link ChronicleMap}, on which {@code acquireUsing()} is queried
+ * @param <V> the value type of {@link ChronicleMap}, on which {@code acquireUsing()} is queried
+ * @see OffHeapUpdatableChronicleMapBuilder#prepareValueBytesOnAcquire(PrepareValueBytes)
+ */
+public interface PrepareValueBytes<K, V> extends Serializable {
 
+    /**
+     * Prepares bytes before assigning them into {@link Byteable}. The given bytes' position
+     * corresponds to the start of value bytes to be assigned into. This method should increment
+     * the given bytes' position by {@linkplain Byteable#maxSize() the value size}. Bytes' contents
+     * outside of this range shouldn't be altered.
+     *
+     * @param bytes the bytes, positioned at the first value byte. Bytes' {@linkplain Bytes#limit(
+     * long) limit} and {@linkplain Bytes#remaining()} are not specified, but it is guaranteed that
+     * the latter is not less than {@linkplain Byteable#maxSize() the value size}.
+     * @param key the key which is absent in the {@link ChronicleMap} during {@link
+     * ChronicleMap#acquireUsing(Object, Object) acquireUsing()} call and value bytes are prepared
+     * for
+     */
     void prepare(Bytes bytes, K key);
 }
diff --git a/src/main/java/net/openhft/chronicle/map/PrepareValueBytesAsWriter.java b/src/main/java/net/openhft/chronicle/map/PrepareValueBytesAsWriter.java
index 13b7712ed..c4f57ff18 100644
--- a/src/main/java/net/openhft/chronicle/map/PrepareValueBytesAsWriter.java
+++ b/src/main/java/net/openhft/chronicle/map/PrepareValueBytesAsWriter.java
@@ -26,10 +26,10 @@ final class PrepareValueBytesAsWriter<K>
         implements BytesWriter<K>, MetaBytesWriter<K, PrepareValueBytesAsWriter<K>>,
         Serializable {
     private static final long serialVersionUID = 0L;
-    private final PrepareValueBytes<K> prepareValueBytes;
+    private final PrepareValueBytes<K, ?> prepareValueBytes;
     private final long valueSize;
 
-    PrepareValueBytesAsWriter(PrepareValueBytes<K> prepareValueBytes, long valueSize) {
+    PrepareValueBytesAsWriter(PrepareValueBytes<K, ?> prepareValueBytes, long valueSize) {
         this.prepareValueBytes = prepareValueBytes;
         this.valueSize = valueSize;
     }
diff --git a/src/main/java/net/openhft/chronicle/map/ZeroOutValueBytes.java b/src/main/java/net/openhft/chronicle/map/ZeroOutValueBytes.java
index 67a4ea96f..a6e2ce845 100644
--- a/src/main/java/net/openhft/chronicle/map/ZeroOutValueBytes.java
+++ b/src/main/java/net/openhft/chronicle/map/ZeroOutValueBytes.java
@@ -18,7 +18,7 @@ package net.openhft.chronicle.map;
 
 import net.openhft.lang.io.Bytes;
 
-final class ZeroOutValueBytes<K> implements PrepareValueBytes<K> {
+final class ZeroOutValueBytes<K, V> implements PrepareValueBytes<K, V> {
     private static final long serialVersionUID = 0L;
     private final long valueSize;
 
@@ -30,5 +30,6 @@ final class ZeroOutValueBytes<K> implements PrepareValueBytes<K> {
     public void prepare(Bytes bytes, K key) {
         long pos = bytes.position();
         bytes.zeroOut(pos, pos + valueSize);
+        bytes.skip(valueSize);
     }
 }