diff --git a/versions-common/src/main/java/org/codehaus/mojo/versions/api/ArtifactVersionsCache.java b/versions-common/src/main/java/org/codehaus/mojo/versions/api/ArtifactVersionsCache.java
index 4eea8923cc..5c242f2cbb 100644
--- a/versions-common/src/main/java/org/codehaus/mojo/versions/api/ArtifactVersionsCache.java
+++ b/versions-common/src/main/java/org/codehaus/mojo/versions/api/ArtifactVersionsCache.java
@@ -19,9 +19,9 @@
  * under the License.
  */
 
-import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.commons.lang3.function.TriFunction;
 import org.apache.commons.lang3.tuple.Triple;
@@ -32,7 +32,7 @@
 public class ArtifactVersionsCache {
     private TriFunction<AbstractVersionDetails, Optional<Segment>, Boolean, ?> cachedFunction;
     private Map<Triple<? extends AbstractVersionDetails, Optional<Segment>, Boolean>, Object> updateCache =
-            new HashMap<>();
+            new ConcurrentHashMap<>();
 
     /**
      * Constructs a new instance given the concrete function for obtaining the details
diff --git a/versions-common/src/main/java/org/codehaus/mojo/versions/api/DefaultVersionsHelper.java b/versions-common/src/main/java/org/codehaus/mojo/versions/api/DefaultVersionsHelper.java
index c027c63177..fdb835a5ee 100644
--- a/versions-common/src/main/java/org/codehaus/mojo/versions/api/DefaultVersionsHelper.java
+++ b/versions-common/src/main/java/org/codehaus/mojo/versions/api/DefaultVersionsHelper.java
@@ -40,6 +40,7 @@
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -150,7 +151,7 @@ RuleSet getRuleSet() {
      *
      * @since 2.12
      */
-    private final Map<String, Rule> artifactBestFitRule = new HashMap<>();
+    private final Map<String, Rule> artifactBestFitRule = new ConcurrentHashMap<>();
 
     private final List<RemoteRepository> remoteProjectRepositories;
 
@@ -441,7 +442,9 @@ protected Rule getBestFitRule(String groupId, String artifactId) {
             bestFit = rule;
         }
 
-        artifactBestFitRule.put(groupArtifactId, bestFit);
+        if (bestFit != null) {
+            artifactBestFitRule.put(groupArtifactId, bestFit);
+        }
         return bestFit;
     }