diff --git a/ethereum/referencetests/src/main/java/org/hyperledger/besu/ethereum/referencetests/ReferenceTestEnv.java b/ethereum/referencetests/src/main/java/org/hyperledger/besu/ethereum/referencetests/ReferenceTestEnv.java
index 507bca6d50..c373f155f0 100644
--- a/ethereum/referencetests/src/main/java/org/hyperledger/besu/ethereum/referencetests/ReferenceTestEnv.java
+++ b/ethereum/referencetests/src/main/java/org/hyperledger/besu/ethereum/referencetests/ReferenceTestEnv.java
@@ -50,7 +50,7 @@ public class ReferenceTestEnv extends BlockHeader {
   @JsonCreator
   public ReferenceTestEnv(
       @JsonProperty("currentCoinbase") final String coinbase,
-      @JsonProperty("currentDifficulty") final String difficulty,
+      @JsonProperty(value = "currentDifficulty", required = false) final String difficulty,
       @JsonProperty("currentGasLimit") final String gasLimit,
       @JsonProperty("currentNumber") final String number,
       @JsonProperty(value = "currentBaseFee", required = false) final String baseFee,
@@ -63,7 +63,7 @@ public class ReferenceTestEnv extends BlockHeader {
         Hash.EMPTY, // transactionsRoot
         Hash.EMPTY, // receiptsRoot
         new LogsBloomFilter(),
-        Difficulty.fromHexString(difficulty),
+        Optional.ofNullable(difficulty).map(Difficulty::fromHexString).orElse(Difficulty.ZERO),
         Long.decode(number),
         Long.decode(gasLimit),
         0L,
diff --git a/ethereum/referencetests/src/reference-test/external-resources b/ethereum/referencetests/src/reference-test/external-resources
index a380655e5f..638da7ca7b 160000
--- a/ethereum/referencetests/src/reference-test/external-resources
+++ b/ethereum/referencetests/src/reference-test/external-resources
@@ -1 +1 @@
-Subproject commit a380655e5ffab1a5ea0f4d860224bdb19013f06a
+Subproject commit 638da7ca7b2ca371ebb33d7d7d2562981aafd9b2
diff --git a/ethereum/referencetests/src/reference-test/java/org/hyperledger/besu/ethereum/mainnet/DifficultyCalculatorTests.java b/ethereum/referencetests/src/reference-test/java/org/hyperledger/besu/ethereum/mainnet/DifficultyCalculatorTests.java
index ab69151b05..9db8268594 100644
--- a/ethereum/referencetests/src/reference-test/java/org/hyperledger/besu/ethereum/mainnet/DifficultyCalculatorTests.java
+++ b/ethereum/referencetests/src/reference-test/java/org/hyperledger/besu/ethereum/mainnet/DifficultyCalculatorTests.java
@@ -62,6 +62,21 @@ public class DifficultyCalculatorTests {
                   .getConfigOptions(),
               EvmConfiguration.DEFAULT)
         ),
+        Arguments.of(
+          "/DifficultyTests/dfGrayGlacier/difficultyGrayGlacierForkBlock.json",
+          MainnetProtocolSchedule.fromConfig(
+              new StubGenesisConfigOptions().grayGlacierBlock(15050000))
+        ),
+        Arguments.of(
+                "/DifficultyTests/dfGrayGlacier/difficultyGrayGlacierTimeDiff1.json",
+                MainnetProtocolSchedule.fromConfig(
+                        new StubGenesisConfigOptions().grayGlacierBlock(15050000))
+        ),
+        Arguments.of(
+                "/DifficultyTests/dfGrayGlacier/difficultyGrayGlacierTimeDiff2.json",
+                MainnetProtocolSchedule.fromConfig(
+                        new StubGenesisConfigOptions().grayGlacierBlock(15050000))
+        ),
         Arguments.of(
           "/DifficultyTests/dfArrowGlacier/difficultyArrowGlacierForkBlock.json",
           MainnetProtocolSchedule.fromConfig(