diff --git a/CHANGES.txt b/CHANGES.txt
index 8a17ee639a..eb07e60cb6 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 5.1
+ * Fix version range check in MessagingService.getVersionOrdinal (CASSANDRA-20842)
  * Allow custom constraints to be loaded via SPI (CASSANDRA-20824)
  * Optimize DataPlacement lookup by ReplicationParams (CASSANDRA-20804)
  * Fix ShortPaxosSimulationTest and AccordSimulationRunner do not execute from the cli (CASSANDRA-20805)
diff --git a/src/java/org/apache/cassandra/net/MessagingService.java b/src/java/org/apache/cassandra/net/MessagingService.java
index e209cdfb27..eef34378ed 100644
--- a/src/java/org/apache/cassandra/net/MessagingService.java
+++ b/src/java/org/apache/cassandra/net/MessagingService.java
@@ -287,7 +287,7 @@ public class MessagingService extends MessagingServiceMBeanImpl implements Messa
         }
     }
     static final int minVersion = Version.values()[0].value;
-    static final int maxVersion = Version.values()[Version.values().length - 1].value;
+    static final int maxOrdinalVersion = Version.values().length - 1;
 
     /**
      * This is an optimisation to speed up the translation of the serialization
@@ -300,8 +300,8 @@ public class MessagingService extends MessagingServiceMBeanImpl implements Messa
     public static int getVersionOrdinal(int version)
     {
         int result = version - minVersion;
-        if (result < 0 || result > maxVersion)
-            throw new IllegalStateException("Unkown serialization version: " + version);
+        if (result < 0 || result > maxOrdinalVersion)
+            throw new IllegalStateException("Unknown serialization version: " + version);
         return result;
     }
 
diff --git a/src/java/org/apache/cassandra/service/accord/CommandsForRanges.java b/src/java/org/apache/cassandra/service/accord/CommandsForRanges.java
index bbbb7679d3..8aa9f3dd88 100644
--- a/src/java/org/apache/cassandra/service/accord/CommandsForRanges.java
+++ b/src/java/org/apache/cassandra/service/accord/CommandsForRanges.java
@@ -33,7 +33,6 @@ import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Consumer;
 import javax.annotation.Nullable;
 
-import accord.api.Journal;
 import accord.api.RoutingKey;
 import accord.local.Command;
 import accord.local.CommandSummaries;
diff --git a/test/unit/org/apache/cassandra/net/MessagingServiceOrdinaryVersionTest.java b/test/unit/org/apache/cassandra/net/MessagingServiceOrdinaryVersionTest.java
index bbe77f5fa6..33a4df3099 100644
--- a/test/unit/org/apache/cassandra/net/MessagingServiceOrdinaryVersionTest.java
+++ b/test/unit/org/apache/cassandra/net/MessagingServiceOrdinaryVersionTest.java
@@ -37,9 +37,16 @@ public class MessagingServiceOrdinaryVersionTest
         MessagingService.getVersionOrdinal(1);
     }
 
+    @Test(expected = IllegalStateException.class)
+    public void checkUnknownSmallVersionJustBeforeTheMinOne()
+    {
+        MessagingService.getVersionOrdinal(MessagingService.minVersion - 1);
+    }
+
     @Test(expected = IllegalStateException.class)
     public void checkUnknownBigVersion()
     {
-        MessagingService.getVersionOrdinal(Byte.MAX_VALUE + 1);
+        int maxVersion = MessagingService.Version.values()[MessagingService.Version.values().length - 1].value;
+        MessagingService.getVersionOrdinal(maxVersion + 1);
     }
 }