diff --git a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
index cf40525c79c1..83bcfd1af5fa 100644
--- a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
+++ b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
@@ -422,9 +422,27 @@ public List<MavenProject> executeForkedExecutions(
                 for (Map.Entry<String, List<MojoExecution>> fork : forkedExecutions.entrySet()) {
                     String projectId = fork.getKey();
 
-                    int index = projectIndex.getIndices().get(projectId);
+                    Integer idx = projectIndex.getIndices().get(projectId);
+                    if (idx == null) {
+                        throw new LifecycleExecutionException(
+                                "Forked execution references project '" + projectId
+                                        + "' which is not in the reactor. This can happen with parallel builds"
+                                        + " (-T) or when extensions modify the session projects.",
+                                mojoExecution,
+                                project);
+                    }
+                    int index = idx;
 
                     MavenProject forkedProject = projectIndex.getProjects().get(projectId);
+                    if (forkedProject == null) {
+                        throw new LifecycleExecutionException(
+                                "Forked execution references project '" + projectId
+                                        + "' which is not in the reactor (no project instance). This can happen"
+                                        + " with parallel builds (-T) or when extensions modify the session"
+                                        + " projects.",
+                                mojoExecution,
+                                project);
+                    }
 
                     forkedProjects.add(forkedProject);
 
diff --git a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/ProjectIndex.java b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/ProjectIndex.java
index 372cbf1b4376..8f87bfafb17b 100644
--- a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/ProjectIndex.java
+++ b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/ProjectIndex.java
@@ -18,9 +18,9 @@
  */
 package org.apache.maven.lifecycle.internal;
 
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.maven.lifecycle.internal.builder.BuilderCommon;
 import org.apache.maven.project.MavenProject;
@@ -43,8 +43,8 @@ public final class ProjectIndex {
     private final Map<String, Integer> indices;
 
     public ProjectIndex(List<MavenProject> projects) {
-        this.projects = new HashMap<>(projects.size() * 2);
-        this.indices = new HashMap<>(projects.size() * 2);
+        this.projects = new ConcurrentHashMap<>(projects.size() * 2);
+        this.indices = new ConcurrentHashMap<>(projects.size() * 2);
 
         for (int i = 0; i < projects.size(); i++) {
             MavenProject project = projects.get(i);