diff --git a/src/main/java/org/qortal/data/block/BlockData.java b/src/main/java/org/qortal/data/block/BlockData.java
index 61d1a7fb..763bca45 100644
--- a/src/main/java/org/qortal/data/block/BlockData.java
+++ b/src/main/java/org/qortal/data/block/BlockData.java
@@ -211,6 +211,14 @@ public class BlockData implements Serializable {
 		this.onlineAccountsSignatures = onlineAccountsSignatures;
 	}
 
+	public int getOnlineAccountsSignaturesCount() {
+		if (this.onlineAccountsSignatures != null && this.onlineAccountsSignatures.length > 0) {
+			// Blocks use a single online accounts signature, so there is no need for this to be dynamic
+			return 1;
+		}
+		return 0;
+	}
+
 	public boolean isTrimmed() {
 		long onlineAccountSignaturesTrimmedTimestamp = NTP.getTime() - BlockChain.getInstance().getOnlineAccountSignaturesMaxLifetime();
 		long currentTrimmableTimestamp = NTP.getTime() - Settings.getInstance().getAtStatesMaxLifetime();
diff --git a/src/main/java/org/qortal/transform/block/BlockTransformer.java b/src/main/java/org/qortal/transform/block/BlockTransformer.java
index 48e79699..9e02a6f5 100644
--- a/src/main/java/org/qortal/transform/block/BlockTransformer.java
+++ b/src/main/java/org/qortal/transform/block/BlockTransformer.java
@@ -235,7 +235,7 @@ public class BlockTransformer extends Transformer {
 			// Online accounts timestamp is only present if there are also signatures
 			onlineAccountsTimestamp = byteBuffer.getLong();
 
-			final int signaturesByteLength = onlineAccountsSignaturesCount * Transformer.SIGNATURE_LENGTH;
+			final int signaturesByteLength = getOnlineAccountSignaturesLength(onlineAccountsSignaturesCount, onlineAccountsCount, timestamp);
 			if (signaturesByteLength > BlockChain.getInstance().getMaxBlockSize())
 				throw new TransformationException("Byte data too long for online accounts signatures");
 
@@ -371,7 +371,7 @@ public class BlockTransformer extends Transformer {
 
 			if (onlineAccountsSignatures != null && onlineAccountsSignatures.length > 0) {
 				// Note: we write the number of signatures, not the number of bytes
-				bytes.write(Ints.toByteArray(onlineAccountsSignatures.length / Transformer.SIGNATURE_LENGTH));
+				bytes.write(Ints.toByteArray(blockData.getOnlineAccountsSignaturesCount()));
 
 				// We only write online accounts timestamp if we have signatures
 				bytes.write(Longs.toByteArray(blockData.getOnlineAccountsTimestamp()));
@@ -511,6 +511,16 @@ public class BlockTransformer extends Transformer {
 
 		return nonces;
 	}
+	public static int getOnlineAccountSignaturesLength(int onlineAccountsSignaturesCount, int onlineAccountCount, long blockTimestamp) {
+		if (blockTimestamp >= BlockChain.getInstance().getOnlineAccountsMemoryPoWTimestamp()) {
+			// Once mempow is active, we expect the online account signatures to be appended with the nonce values
+			return (onlineAccountsSignaturesCount * Transformer.SIGNATURE_LENGTH) + (onlineAccountCount * INT_LENGTH);
+		}
+		else {
+			// Before mempow, only the online account signatures were included (which will likely be a single signature)
+			return onlineAccountsSignaturesCount * Transformer.SIGNATURE_LENGTH;
+		}
+	}
 
 	public static byte[] extract(byte[] input, int pos, int length) {
 		byte[] output = new byte[length];