diff --git a/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt b/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
index 89e4f8d..4396fa8 100644
--- a/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
+++ b/src/commonMain/kotlin/net.sergeych.bipack/BipackEncoder.kt
@@ -63,7 +63,7 @@ class BipackEncoder(val output: DataSink) : AbstractEncoder() {
         if (fixedSize < 0)
             encodeUInt(collectionSize.toUInt())
         else if (collectionSize != fixedSize) {
-            throw IllegalArgumentException("collection size is $collectionSize while fixed size of $fixedSize is required")
+            throw WrongCollectionSize("collection size is $collectionSize while fixed size of $fixedSize is required")
         }
         return this
     }
diff --git a/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt b/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
index 3808179..370a8d2 100644
--- a/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
+++ b/src/commonMain/kotlin/net.sergeych.bipack/annotations.kt
@@ -44,16 +44,24 @@ annotation class CrcProtected
  * it is the conly way to tell the serialized to use more compact unsigned variable length encoding.
  */
 @SerialInfo
-@Target(AnnotationTarget.FIELD,AnnotationTarget.PROPERTY)
+@Target(AnnotationTarget.FIELD, AnnotationTarget.PROPERTY)
 annotation class Unsigned
 
+/**
+ * Use it with collection of fixed size, like hash digest, key bits and so on, by not storing collection
+ * size. It effectively reduced packed size to at least one byte. depending on the actual
+ * collection size. As for nowonly collection types (e.g. ByteArray, List<T>m etc) are supported.
+ * Note that if the actual collection size differs from [size], [BipackEncoder] will throw
+ * [WrongCollectionSize] while encoding it.
+ */
 @SerialInfo
-@Target(AnnotationTarget.FIELD,AnnotationTarget.PROPERTY)
+@Target(AnnotationTarget.FIELD, AnnotationTarget.PROPERTY)
 annotation class FixedSize(val size: Int)
 
 open class InvalidFrameException(reason: String) : Exception(reason)
 class InvalidFrameHeaderException(reason: String = "Frame header does not match") : InvalidFrameException(reason)
 class InvalidFrameCRCException : InvalidFrameException("Checksum CRC32 failed")
 
+class WrongCollectionSize(reason: String) : Exception(reason)