diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModel.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModel.java
index f9a0a81700f..7cdf480e056 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModel.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModel.java
@@ -7,6 +7,7 @@
 package org.elasticsearch.xpack.core.ml.inference.trainedmodel.inference;
 
 import org.apache.lucene.util.Accountable;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.Numbers;
 import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.xcontent.ConstructingObjectParser;
@@ -60,12 +61,16 @@ public class TreeInferenceModel implements InferenceModel {
     private static final ConstructingObjectParser<TreeInferenceModel, Void> PARSER = new ConstructingObjectParser<>(
         "tree_inference_model",
         true,
-        a -> new TreeInferenceModel((List<String>)a[0], (List<NodeBuilder>)a[1], TargetType.fromString((String)a[2]), (List<String>)a[3]));
+        a -> new TreeInferenceModel(
+            (List<String>)a[0],
+            (List<NodeBuilder>)a[1],
+            a[2] == null ? null : TargetType.fromString((String)a[2]),
+            (List<String>)a[3]));
 
     static {
         PARSER.declareStringArray(constructorArg(), FEATURE_NAMES);
         PARSER.declareObjectArray(constructorArg(), NodeBuilder.PARSER::apply, TREE_STRUCTURE);
-        PARSER.declareString(constructorArg(), TARGET_TYPE);
+        PARSER.declareString(optionalConstructorArg(), TARGET_TYPE);
         PARSER.declareStringArray(optionalConstructorArg(), CLASSIFICATION_LABELS);
     }
 
@@ -82,13 +87,16 @@ public class TreeInferenceModel implements InferenceModel {
     private final int leafSize;
     private volatile boolean preparedForInference = false;
 
-    TreeInferenceModel(List<String> featureNames, List<NodeBuilder> nodes, TargetType targetType, List<String> classificationLabels) {
+    TreeInferenceModel(List<String> featureNames,
+                       List<NodeBuilder> nodes,
+                       @Nullable TargetType targetType,
+                       List<String> classificationLabels) {
         this.featureNames = ExceptionsHelper.requireNonNull(featureNames, FEATURE_NAMES).toArray(new String[0]);
         if(ExceptionsHelper.requireNonNull(nodes, TREE_STRUCTURE).size() == 0) {
             throw new IllegalArgumentException("[tree_structure] must not be empty");
         }
         this.nodes = nodes.stream().map(NodeBuilder::build).toArray(Node[]::new);
-        this.targetType = ExceptionsHelper.requireNonNull(targetType, TARGET_TYPE);
+        this.targetType = targetType == null ? TargetType.REGRESSION : targetType;
         this.classificationLabels = classificationLabels == null ? null : Collections.unmodifiableList(classificationLabels);
         this.highOrderCategory = maxLeafValue();
         int leafSize = 1;
@@ -357,7 +365,7 @@ public class TreeInferenceModel implements InferenceModel {
         return Math.max(depthLeft, depthRight) + 1;
     }
 
-    private static class NodeBuilder {
+    static class NodeBuilder {
 
         private static final ObjectParser<NodeBuilder, Void> PARSER = new ObjectParser<>(
             "tree_inference_model_node",
diff --git a/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModelTests.java b/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModelTests.java
index e0d40848376..995bba94024 100644
--- a/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModelTests.java
+++ b/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/inference/trainedmodel/inference/TreeInferenceModelTests.java
@@ -58,6 +58,15 @@ public class TreeInferenceModelTests extends ESTestCase {
         return new NamedXContentRegistry(namedXContent);
     }
 
+    public void testCtorWithNullTargetType() {
+        TreeInferenceModel treeInferenceModel = new TreeInferenceModel(
+            Collections.emptyList(),
+            Collections.singletonList(new TreeInferenceModel.NodeBuilder().setLeafValue(new double[]{1.0}).setNumberSamples(100L)),
+            null,
+            Collections.emptyList());
+        assertThat(treeInferenceModel.targetType(), equalTo(TargetType.REGRESSION));
+    }
+
     public void testSerializationFromEnsemble() throws Exception {
         for (int i = 0; i < NUMBER_OF_TEST_RUNS; ++i) {
             Tree tree = TreeTests.createRandom(randomFrom(TargetType.values()));