diff --git a/spark-operator-api/src/main/java/org/apache/spark/k8s/operator/spec/ClusterSpec.java b/spark-operator-api/src/main/java/org/apache/spark/k8s/operator/spec/ClusterSpec.java
index 6786abbc..408f8d62 100644
--- a/spark-operator-api/src/main/java/org/apache/spark/k8s/operator/spec/ClusterSpec.java
+++ b/spark-operator-api/src/main/java/org/apache/spark/k8s/operator/spec/ClusterSpec.java
@@ -36,7 +36,7 @@
 @JsonInclude(JsonInclude.Include.NON_NULL)
 @JsonIgnoreProperties(ignoreUnknown = true)
 public class ClusterSpec extends BaseSpec {
-  @Required protected RuntimeVersions runtimeVersions;
+  @Required @Builder.Default protected RuntimeVersions runtimeVersions = new RuntimeVersions();
 
   @Required @Builder.Default
   protected ClusterTolerations clusterTolerations = new ClusterTolerations();
diff --git a/spark-operator-api/src/test/java/org/apache/spark/k8s/operator/spec/ClusterSpecTest.java b/spark-operator-api/src/test/java/org/apache/spark/k8s/operator/spec/ClusterSpecTest.java
index 2801860a..b96455a1 100644
--- a/spark-operator-api/src/test/java/org/apache/spark/k8s/operator/spec/ClusterSpecTest.java
+++ b/spark-operator-api/src/test/java/org/apache/spark/k8s/operator/spec/ClusterSpecTest.java
@@ -20,7 +20,6 @@
 package org.apache.spark.k8s.operator.spec;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertNull;
 
 import org.junit.jupiter.api.Test;
 
@@ -35,7 +34,9 @@ void testBuilder() {
   @Test
   void testInitSpecWithDefaults() {
     ClusterSpec spec1 = new ClusterSpec();
-    assertNull(spec1.runtimeVersions);
+    assertEquals(null, spec1.runtimeVersions.jdkVersion);
+    assertEquals(null, spec1.runtimeVersions.scalaVersion);
+    assertEquals(null, spec1.runtimeVersions.sparkVersion);
     assertEquals(0, spec1.clusterTolerations.instanceConfig.initWorkers);
     assertEquals(0, spec1.clusterTolerations.instanceConfig.minWorkers);
     assertEquals(0, spec1.clusterTolerations.instanceConfig.maxWorkers);