diff --git a/server/app/com/xsn/explorer/models/Block.scala b/server/app/com/xsn/explorer/models/Block.scala
index 7976e30..14cb582 100644
--- a/server/app/com/xsn/explorer/models/Block.scala
+++ b/server/app/com/xsn/explorer/models/Block.scala
@@ -11,8 +11,8 @@ import play.api.libs.json._
  */
 case class Block(
     hash: Blockhash,
-    previousBlockhash: Blockhash,
-    nextBlockhash: Blockhash,
+    previousBlockhash: Option[Blockhash], // first block doesn't have a previous block
+    nextBlockhash: Option[Blockhash], // last block doesn't have a next block
     merkleRoot: Blockhash,
     transactions: List[TransactionId],
     confirmations: Confirmations,
@@ -24,14 +24,30 @@ case class Block(
     nonce: Int,
     bits: String,
     chainwork: String,
-    difficulty: BigDecimal
-)
+    difficulty: BigDecimal,
+    tposContract: Option[String]) {
+
+  /**
+   * Every block until 75 is PoW.
+   */
+  def isPoW: Boolean = height.int <= 75
+
+  /**
+   * From block 76, we have just PoW or TPoS
+   */
+  def isPoS: Boolean = !isPoW && tposContract.isEmpty
+
+  /**
+   * TPoS blocks need a tposcontract
+   */
+  def isTPoS: Boolean = !isPoW && tposContract.nonEmpty
+}
 
 object Block {
   implicit val reads: Reads[Block] = {
     val builder = (__ \ 'hash).read[Blockhash] and
-        (__ \ 'previousblockhash).read[Blockhash] and
-        (__ \ 'nextblockhash).read[Blockhash] and
+        (__ \ 'previousblockhash).readNullable[Blockhash] and
+        (__ \ 'nextblockhash).readNullable[Blockhash] and
         (__ \ 'merkleroot).read[Blockhash] and
         (__ \ 'tx).read[List[TransactionId]] and
         (__ \ 'confirmations).read[Confirmations] and
@@ -43,16 +59,17 @@ object Block {
         (__ \ 'nonce).read[Int] and
         (__ \ 'bits).read[String] and
         (__ \ 'chainwork).read[String] and
-        (__ \ 'difficulty).read[BigDecimal]
+        (__ \ 'difficulty).read[BigDecimal] and
+        (__ \ 'tposcontract).readNullable[String]
 
     builder.apply { (hash, previous, next, root, transactions,
         confirmations, size, height, version, time,
-        medianTime, nonce, bits, chainwork, difficulty) =>
+        medianTime, nonce, bits, chainwork, difficulty, tposContract) =>
 
       Block(
         hash, previous, next, root, transactions,
         confirmations, size, height, version, time,
-        medianTime, nonce, bits, chainwork, difficulty)
+        medianTime, nonce, bits, chainwork, difficulty, tposContract)
     }
   }
 
diff --git a/server/test/controllers/BlocksControllerSpec.scala b/server/test/controllers/BlocksControllerSpec.scala
index 97ddc7a..03cc5b5 100644
--- a/server/test/controllers/BlocksControllerSpec.scala
+++ b/server/test/controllers/BlocksControllerSpec.scala
@@ -139,8 +139,8 @@ class BlocksControllerSpec extends MyAPISpec {
       (jsonBlock \ "merkleRoot").as[Blockhash] mustEqual block.merkleRoot
       (jsonBlock \ "version").as[Long] mustEqual block.version
       (jsonBlock \ "nonce").as[Int] mustEqual block.nonce
-      (jsonBlock \ "previousBlockhash").as[Blockhash] mustEqual block.previousBlockhash
-      (jsonBlock \ "nextBlockhash").as[Blockhash] mustEqual block.nextBlockhash
+      (jsonBlock \ "previousBlockhash").asOpt[Blockhash] mustEqual block.previousBlockhash
+      (jsonBlock \ "nextBlockhash").asOpt[Blockhash] mustEqual block.nextBlockhash
 
       val jsonCoinstake = (jsonRewards \ "coinstake").as[JsValue]
       (jsonCoinstake \ "address").as[String] mustEqual "XgEGH3y7RfeKEdn2hkYEvBnrnmGBr7zvjL"
@@ -173,8 +173,8 @@ class BlocksControllerSpec extends MyAPISpec {
       (jsonBlock \ "merkleRoot").as[Blockhash] mustEqual block.merkleRoot
       (jsonBlock \ "version").as[Long] mustEqual block.version
       (jsonBlock \ "nonce").as[Int] mustEqual block.nonce
-      (jsonBlock \ "previousBlockhash").as[Blockhash] mustEqual block.previousBlockhash
-      (jsonBlock \ "nextBlockhash").as[Blockhash] mustEqual block.nextBlockhash
+      (jsonBlock \ "previousBlockhash").asOpt[Blockhash] mustEqual block.previousBlockhash
+      (jsonBlock \ "nextBlockhash").asOpt[Blockhash] mustEqual block.nextBlockhash
 
       val jsonCoinstake = (jsonRewards \ "coinstake").as[JsValue]
       (jsonCoinstake \ "address").as[String] mustEqual "XgEGH3y7RfeKEdn2hkYEvBnrnmGBr7zvjL"
@@ -245,8 +245,9 @@ class BlocksControllerSpec extends MyAPISpec {
       bits = "1d011212",
       chainwork = "00000000000000000000000000000000000000000000000000000084c71ff420",
       difficulty = BigDecimal("0.9340526210769362"),
-      previousBlockhash = Blockhash.from("000003dc4c2fc449dededaaad6efc33ce1b64b88a060652dc47edc63d6d6b524").get,
-      nextBlockhash = Blockhash.from("000000125c06cedf38b07bff174bdb61027935dbcb34831d28cff40bedb519d5").get
+      previousBlockhash = Some(Blockhash.from("000003dc4c2fc449dededaaad6efc33ce1b64b88a060652dc47edc63d6d6b524").get),
+      nextBlockhash = Some(Blockhash.from("000000125c06cedf38b07bff174bdb61027935dbcb34831d28cff40bedb519d5").get),
+      tposContract = None
     )
   }
 }