diff --git a/src/main/java/org/apache/xmlbeans/impl/validator/Validator.java b/src/main/java/org/apache/xmlbeans/impl/validator/Validator.java
index da7b8bfec..5ffeada18 100644
--- a/src/main/java/org/apache/xmlbeans/impl/validator/Validator.java
+++ b/src/main/java/org/apache/xmlbeans/impl/validator/Validator.java
@@ -373,7 +373,7 @@ private void beginEvent(Event event) {
             }
 
             if (elementType.blockExtension()) {
-                for (SchemaType t = xsiType; !t.equals(elementType);
+                for (SchemaType t = xsiType; t != null && !t.equals(elementType);
                      t = t.getBaseType()) {
                     if (t.getDerivationType() == SchemaType.DT_EXTENSION) {
                         emitFieldError(event, XmlErrorCodes.ELEM_LOCALLY_VALID$XSI_TYPE_BLOCK_EXTENSION,
@@ -388,7 +388,7 @@ private void beginEvent(Event event) {
             }
 
             if (elementType.blockRestriction()) {
-                for (SchemaType t = xsiType; !t.equals(elementType);
+                for (SchemaType t = xsiType; t != null && !t.equals(elementType);
                      t = t.getBaseType()) {
                     if (t.getDerivationType() == SchemaType.DT_RESTRICTION) {
                         emitFieldError(event, XmlErrorCodes.ELEM_LOCALLY_VALID$XSI_TYPE_BLOCK_RESTRICTION,
@@ -406,7 +406,7 @@ private void beginEvent(Event event) {
                 _localElement = sle;
 
                 if (sle.blockExtension() || sle.blockRestriction()) {
-                    for (SchemaType t = xsiType; !t.equals(elementType);
+                    for (SchemaType t = xsiType; t != null && !t.equals(elementType);
                          t = t.getBaseType()) {
                         if ((t.getDerivationType() == SchemaType.DT_RESTRICTION && sle.blockRestriction()) ||
                             (t.getDerivationType() == SchemaType.DT_EXTENSION && sle.blockExtension())) {
diff --git a/src/test/java/misc/checkin/XsiTypeBlockValidateTest.java b/src/test/java/misc/checkin/XsiTypeBlockValidateTest.java
new file mode 100644
index 000000000..53821f74a
--- /dev/null
+++ b/src/test/java/misc/checkin/XsiTypeBlockValidateTest.java
@@ -0,0 +1,97 @@
+/*   Copyright 2004 The Apache Software Foundation
+ *
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+package misc.checkin;
+
+import org.apache.xmlbeans.SchemaTypeLoader;
+import org.apache.xmlbeans.XmlBeans;
+import org.apache.xmlbeans.XmlError;
+import org.apache.xmlbeans.XmlObject;
+import org.apache.xmlbeans.XmlOptions;
+import org.apache.xmlbeans.impl.xb.xsdschema.SchemaDocument;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class XsiTypeBlockValidateTest {
+
+    private static final String UNION_MEMBER_INSTANCE =
+        "<t:root xmlns:t='urn:t' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' " +
+        "xmlns:xs='http://www.w3.org/2001/XMLSchema' xsi:type='xs:int'>5</t:root>";
+
+    private static XmlObject parse(String xsd, String instance) throws Exception {
+        SchemaTypeLoader loader = XmlBeans.loadXsd(new XmlObject[]{SchemaDocument.Factory.parse(xsd)});
+        return loader.parse(instance, null, null);
+    }
+
+    @Test
+    void unionMemberSubstitutionUnderElementBlock() throws Exception {
+        // xs:int is validly derived from the union, and block only excludes extension
+        String xsd =
+            "<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema' " +
+            "xmlns:t='urn:t' targetNamespace='urn:t'>" +
+            "  <xs:simpleType name='u'><xs:union memberTypes='xs:int xs:string'/></xs:simpleType>" +
+            "  <xs:element name='root' type='t:u' block='extension'/>" +
+            "</xs:schema>";
+
+        XmlObject doc = parse(xsd, UNION_MEMBER_INSTANCE);
+        List<XmlError> errors = new ArrayList<>();
+        assertTrue(doc.validate(new XmlOptions().setErrorListener(errors)), errors.toString());
+    }
+
+    @Test
+    void unionMemberSubstitutionUnderTypeBlock() throws Exception {
+        String xsd =
+            "<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema' " +
+            "xmlns:t='urn:t' targetNamespace='urn:t'>" +
+            "  <xs:simpleType name='u'><xs:union memberTypes='xs:int xs:string'/></xs:simpleType>" +
+            "  <xs:complexType name='ct' block='extension'>" +
+            "    <xs:simpleContent><xs:extension base='t:u'/></xs:simpleContent>" +
+            "  </xs:complexType>" +
+            "  <xs:element name='root' type='t:ct'/>" +
+            "</xs:schema>";
+
+        XmlObject doc = parse(xsd, UNION_MEMBER_INSTANCE);
+        assertDoesNotThrow(() -> doc.validate(new XmlOptions().setErrorListener(new ArrayList<>())));
+    }
+
+    @Test
+    void blockedExtensionIsStillReported() throws Exception {
+        String xsd =
+            "<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema' " +
+            "xmlns:t='urn:t' targetNamespace='urn:t'>" +
+            "  <xs:complexType name='base'><xs:sequence/></xs:complexType>" +
+            "  <xs:complexType name='ext'>" +
+            "    <xs:complexContent><xs:extension base='t:base'><xs:sequence/></xs:extension></xs:complexContent>" +
+            "  </xs:complexType>" +
+            "  <xs:element name='root' type='t:base' block='extension'/>" +
+            "</xs:schema>";
+
+        XmlObject doc = parse(xsd,
+            "<t:root xmlns:t='urn:t' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' " +
+            "xsi:type='t:ext'/>");
+
+        List<XmlError> errors = new ArrayList<>();
+        assertFalse(doc.validate(new XmlOptions().setErrorListener(errors)));
+        assertEquals(1, errors.size(), errors.toString());
+        assertEquals("cvc-elt.4.3d", errors.get(0).getErrorCode());
+    }
+}