diff --git a/CHANGES.txt b/CHANGES.txt
index 4c9649ae4b..6fd1abe903 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 5.0.9
+ * Fix RequestFailureReason serializer and nits in a few others (CASSANDRA-21437)
  * Remove golang dependency in gen-doc and replace with python implementation (CASSANDRA-21432)
  * Use estimated compressed size for tables to check if there is enough free space for a compaction (CASSANDRA-21245)
  * Fix failing select on system_views.settings for non-string keys (CASSANDRA-21348)
diff --git a/src/java/org/apache/cassandra/cache/SerializingCacheProvider.java b/src/java/org/apache/cassandra/cache/SerializingCacheProvider.java
index 56393c4b72..6d236a4010 100644
--- a/src/java/org/apache/cassandra/cache/SerializingCacheProvider.java
+++ b/src/java/org/apache/cassandra/cache/SerializingCacheProvider.java
@@ -58,7 +58,7 @@ public class SerializingCacheProvider implements CacheProvider<RowCacheKey, IRow
 
         public long serializedSize(IRowCacheEntry entry)
         {
-            int size = TypeSizes.sizeof(true);
+            long size = TypeSizes.sizeof(true);
             if (entry instanceof RowCacheSentinel)
                 size += TypeSizes.sizeof(((RowCacheSentinel) entry).sentinelId);
             else
diff --git a/src/java/org/apache/cassandra/dht/AbstractBounds.java b/src/java/org/apache/cassandra/dht/AbstractBounds.java
index 7a603b0a5d..61fd50c830 100644
--- a/src/java/org/apache/cassandra/dht/AbstractBounds.java
+++ b/src/java/org/apache/cassandra/dht/AbstractBounds.java
@@ -231,9 +231,9 @@ public abstract class AbstractBounds<T extends RingPosition<T>> implements Seria
         public long serializedSize(AbstractBounds<T> ab, int version)
         {
             // !WARNING! See serialize method above for why we still need to have that condition.
-            int size = version < MessagingService.VERSION_30
-                     ? TypeSizes.sizeof(kindInt(ab))
-                     : 1;
+            long size = version < MessagingService.VERSION_30
+                      ? TypeSizes.sizeof(kindInt(ab))
+                      : 1;
             size += serializer.serializedSize(ab.left, version);
             size += serializer.serializedSize(ab.right, version);
             return size;
diff --git a/src/java/org/apache/cassandra/exceptions/RequestFailureReason.java b/src/java/org/apache/cassandra/exceptions/RequestFailureReason.java
index ae55661045..5e9900280c 100644
--- a/src/java/org/apache/cassandra/exceptions/RequestFailureReason.java
+++ b/src/java/org/apache/cassandra/exceptions/RequestFailureReason.java
@@ -96,22 +96,25 @@ public enum RequestFailureReason
         {
         }
 
+        @Override
         public void serialize(RequestFailureReason reason, DataOutputPlus out, int version) throws IOException
         {
             assert version >= VERSION_40;
             out.writeUnsignedVInt32(reason.code);
         }
 
+        @Override
         public RequestFailureReason deserialize(DataInputPlus in, int version) throws IOException
         {
             assert version >= VERSION_40;
             return fromCode(in.readUnsignedVInt32());
         }
 
+        @Override
         public long serializedSize(RequestFailureReason reason, int version)
         {
             assert version >= VERSION_40;
-            return VIntCoding.computeVIntSize(reason.code);
+            return VIntCoding.computeUnsignedVIntSize(reason.code);
         }
     }
 }
diff --git a/src/java/org/apache/cassandra/gms/GossipDigestAck.java b/src/java/org/apache/cassandra/gms/GossipDigestAck.java
index 26494eaba9..93488ae11c 100644
--- a/src/java/org/apache/cassandra/gms/GossipDigestAck.java
+++ b/src/java/org/apache/cassandra/gms/GossipDigestAck.java
@@ -60,6 +60,7 @@ public class GossipDigestAck
 
 class GossipDigestAckSerializer implements IVersionedSerializer<GossipDigestAck>
 {
+    @Override
     public void serialize(GossipDigestAck gDigestAckMessage, DataOutputPlus out, int version) throws IOException
     {
         GossipDigestSerializationHelper.serialize(gDigestAckMessage.gDigestList, out, version);
@@ -72,6 +73,7 @@ class GossipDigestAckSerializer implements IVersionedSerializer<GossipDigestAck>
         }
     }
 
+    @Override
     public GossipDigestAck deserialize(DataInputPlus in, int version) throws IOException
     {
         List<GossipDigest> gDigestList = GossipDigestSerializationHelper.deserialize(in, version);
@@ -87,9 +89,10 @@ class GossipDigestAckSerializer implements IVersionedSerializer<GossipDigestAck>
         return new GossipDigestAck(gDigestList, epStateMap);
     }
 
+    @Override
     public long serializedSize(GossipDigestAck ack, int version)
     {
-        int size = GossipDigestSerializationHelper.serializedSize(ack.gDigestList, version);
+        long size = GossipDigestSerializationHelper.serializedSize(ack.gDigestList, version);
         size += TypeSizes.sizeof(ack.epStateMap.size());
         for (Map.Entry<InetAddressAndPort, EndpointState> entry : ack.epStateMap.entrySet())
             size += inetAddressAndPortSerializer.serializedSize(entry.getKey(), version)
diff --git a/src/java/org/apache/cassandra/gms/GossipDigestSyn.java b/src/java/org/apache/cassandra/gms/GossipDigestSyn.java
index 7c2ae945c8..9e4fd0907e 100644
--- a/src/java/org/apache/cassandra/gms/GossipDigestSyn.java
+++ b/src/java/org/apache/cassandra/gms/GossipDigestSyn.java
@@ -70,9 +70,9 @@ class GossipDigestSerializationHelper
         return gDigests;
     }
 
-    static int serializedSize(List<GossipDigest> digests, int version)
+    static long serializedSize(List<GossipDigest> digests, int version)
     {
-        int size = TypeSizes.sizeof(digests.size());
+        long size = TypeSizes.sizeof(digests.size());
         for (GossipDigest digest : digests)
             size += GossipDigest.serializer.serializedSize(digest, version);
         return size;
diff --git a/src/java/org/apache/cassandra/net/CustomParamsSerializer.java b/src/java/org/apache/cassandra/net/CustomParamsSerializer.java
index c6c72fe6cb..ae03072001 100644
--- a/src/java/org/apache/cassandra/net/CustomParamsSerializer.java
+++ b/src/java/org/apache/cassandra/net/CustomParamsSerializer.java
@@ -49,7 +49,7 @@ class CustomParamsSerializer implements IVersionedSerializer<Map<String,byte[]>>
     @Override
     public long serializedSize(Map<String, byte[]> t, int version)
     {
-        int size = TypeSizes.sizeofUnsignedVInt(t.size());
+        long size = TypeSizes.sizeofUnsignedVInt(t.size());
         for (Map.Entry<String,byte[]> e : t.entrySet())
         {
             size += TypeSizes.sizeof(e.getKey());
diff --git a/src/java/org/apache/cassandra/repair/RepairJobDesc.java b/src/java/org/apache/cassandra/repair/RepairJobDesc.java
index dba336b5e7..a24e045c37 100644
--- a/src/java/org/apache/cassandra/repair/RepairJobDesc.java
+++ b/src/java/org/apache/cassandra/repair/RepairJobDesc.java
@@ -152,7 +152,7 @@ public class RepairJobDesc
 
         public long serializedSize(RepairJobDesc desc, int version)
         {
-            int size = TypeSizes.sizeof(desc.parentSessionId != null);
+            long size = TypeSizes.sizeof(desc.parentSessionId != null);
             if (desc.parentSessionId != null)
                 size += TimeUUID.sizeInBytes();
             size += TimeUUID.sizeInBytes();
diff --git a/src/java/org/apache/cassandra/schema/SchemaMutationsSerializer.java b/src/java/org/apache/cassandra/schema/SchemaMutationsSerializer.java
index ba65c0d0c9..31e3c06adc 100644
--- a/src/java/org/apache/cassandra/schema/SchemaMutationsSerializer.java
+++ b/src/java/org/apache/cassandra/schema/SchemaMutationsSerializer.java
@@ -32,6 +32,7 @@ public class SchemaMutationsSerializer implements IVersionedSerializer<Collectio
 {
     public static final SchemaMutationsSerializer instance = new SchemaMutationsSerializer();
 
+    @Override
     public void serialize(Collection<Mutation> schema, DataOutputPlus out, int version) throws IOException
     {
         out.writeInt(schema.size());
@@ -39,6 +40,7 @@ public class SchemaMutationsSerializer implements IVersionedSerializer<Collectio
             Mutation.serializer.serialize(mutation, out, version);
     }
 
+    @Override
     public Collection<Mutation> deserialize(DataInputPlus in, int version) throws IOException
     {
         int count = in.readInt();
@@ -50,9 +52,10 @@ public class SchemaMutationsSerializer implements IVersionedSerializer<Collectio
         return schema;
     }
 
+    @Override
     public long serializedSize(Collection<Mutation> schema, int version)
     {
-        int size = TypeSizes.sizeof(schema.size());
+        long size = TypeSizes.sizeof(schema.size());
         for (Mutation mutation : schema)
             size += mutation.serializedSize(version);
         return size;
diff --git a/src/java/org/apache/cassandra/streaming/StreamRequest.java b/src/java/org/apache/cassandra/streaming/StreamRequest.java
index dba67917b9..656403812c 100644
--- a/src/java/org/apache/cassandra/streaming/StreamRequest.java
+++ b/src/java/org/apache/cassandra/streaming/StreamRequest.java
@@ -76,6 +76,7 @@ public class StreamRequest
 
     public static class StreamRequestSerializer implements IVersionedSerializer<StreamRequest>
     {
+        @Override
         public void serialize(StreamRequest request, DataOutputPlus out, int version) throws IOException
         {
             out.writeUTF(request.keyspace);
@@ -100,6 +101,7 @@ public class StreamRequest
             }
         }
 
+        @Override
         public StreamRequest deserialize(DataInputPlus in, int version) throws IOException
         {
             String keyspace = in.readUTF();
@@ -131,13 +133,14 @@ public class StreamRequest
             return replicas.build();
         }
 
+        @Override
         public long serializedSize(StreamRequest request, int version)
         {
-            int size = TypeSizes.sizeof(request.keyspace);
+            long size = TypeSizes.sizeof(request.keyspace);
             size += TypeSizes.sizeof(request.columnFamilies.size());
             size += inetAddressAndPortSerializer.serializedSize(request.full.endpoint(), version);
-            size += replicasSerializedSize(request.transientReplicas, version);
             size += replicasSerializedSize(request.full, version);
+            size += replicasSerializedSize(request.transientReplicas, version);
             for (String cf : request.columnFamilies)
                 size += TypeSizes.sizeof(cf);
             return size;
diff --git a/src/java/org/apache/cassandra/utils/BloomFilterSerializer.java b/src/java/org/apache/cassandra/utils/BloomFilterSerializer.java
index 91ec13f53c..fc478b6bf8 100644
--- a/src/java/org/apache/cassandra/utils/BloomFilterSerializer.java
+++ b/src/java/org/apache/cassandra/utils/BloomFilterSerializer.java
@@ -64,7 +64,7 @@ public final class BloomFilterSerializer implements IGenericSerializer<BloomFilt
     @Override
     public long serializedSize(BloomFilter bf)
     {
-        int size = TypeSizes.sizeof(bf.hashCount); // hash count
+        long size = TypeSizes.sizeof(bf.hashCount); // hash count
         size += bf.bitset.serializedSize();
         return size;
     }
diff --git a/src/java/org/apache/cassandra/utils/EstimatedHistogram.java b/src/java/org/apache/cassandra/utils/EstimatedHistogram.java
index 198f92286f..3c4c19f5ea 100644
--- a/src/java/org/apache/cassandra/utils/EstimatedHistogram.java
+++ b/src/java/org/apache/cassandra/utils/EstimatedHistogram.java
@@ -429,7 +429,7 @@ public class EstimatedHistogram implements DoubleToLongFunction
 
         public long serializedSize(EstimatedHistogram eh)
         {
-            int size = 0;
+            long size = 0;
 
             long[] offsets = eh.getBucketOffsets();
             long[] buckets = eh.getBuckets(false);
diff --git a/src/java/org/apache/cassandra/utils/RangesSerializer.java b/src/java/org/apache/cassandra/utils/RangesSerializer.java
index 5707503f6b..a7cab8fc42 100644
--- a/src/java/org/apache/cassandra/utils/RangesSerializer.java
+++ b/src/java/org/apache/cassandra/utils/RangesSerializer.java
@@ -65,9 +65,9 @@ public class RangesSerializer implements IVersionedSerializer<Collection<Range<T
     @Override
     public long serializedSize(Collection<Range<Token>> ranges, int version)
     {
-        int size = TypeSizes.sizeof(ranges.size());
+        long size = TypeSizes.sizeof(ranges.size());
         if (ranges.size() > 0)
-            size += ranges.size() * 2 * Token.serializer.serializedSize(ranges.iterator().next().left, version);
+            size += ranges.size() * 2L * Token.serializer.serializedSize(ranges.iterator().next().left, version);
         return size;
     }
 }