diff --git a/hibernate-core/src/main/java/org/hibernate/query/sqm/tree/select/SqmQueryGroup.java b/hibernate-core/src/main/java/org/hibernate/query/sqm/tree/select/SqmQueryGroup.java
index f90c532e59b7..9f2fdd738d98 100644
--- a/hibernate-core/src/main/java/org/hibernate/query/sqm/tree/select/SqmQueryGroup.java
+++ b/hibernate-core/src/main/java/org/hibernate/query/sqm/tree/select/SqmQueryGroup.java
@@ -170,7 +170,8 @@ private void validateQueryGroupFetchStructure(List<? extends SqmTypedNode<?>> ty
 				for ( int j = 0; j < firstSelectionSize; j++ ) {
 					final SqmTypedNode<?> firstSqmSelection = typedNodes.get( j );
 					final JavaType<?> firstJavaType = firstSqmSelection.getNodeJavaType();
-					if ( firstJavaType != selections.get( j ).getNodeJavaType() ) {
+					final JavaType<?> nodeJavaType = selections.get( j ).getNodeJavaType();
+					if ( nodeJavaType != null && firstJavaType != null && firstJavaType != nodeJavaType ) {
 						throw new SemanticException(
 								"Select items of the same index must have the same java type across all query parts"
 						);
diff --git a/hibernate-core/src/main/java/org/hibernate/sql/results/jdbc/internal/JdbcValuesMappingProducerProviderStandard.java b/hibernate-core/src/main/java/org/hibernate/sql/results/jdbc/internal/JdbcValuesMappingProducerProviderStandard.java
index fa95470f6d4f..151bdb7fc1d8 100644
--- a/hibernate-core/src/main/java/org/hibernate/sql/results/jdbc/internal/JdbcValuesMappingProducerProviderStandard.java
+++ b/hibernate-core/src/main/java/org/hibernate/sql/results/jdbc/internal/JdbcValuesMappingProducerProviderStandard.java
@@ -9,9 +9,15 @@
 import org.hibernate.engine.spi.SessionFactoryImplementor;
 import org.hibernate.query.results.ResultSetMapping;
 import org.hibernate.query.results.ResultSetMappingImpl;
+import org.hibernate.sql.ast.spi.SqlSelection;
+import org.hibernate.sql.ast.tree.select.QueryGroup;
+import org.hibernate.sql.ast.tree.select.QueryPart;
 import org.hibernate.sql.ast.tree.select.SelectStatement;
 import org.hibernate.sql.results.jdbc.spi.JdbcValuesMappingProducer;
 import org.hibernate.sql.results.jdbc.spi.JdbcValuesMappingProducerProvider;
+import org.hibernate.type.descriptor.jdbc.NullJdbcType;
+
+import java.util.List;
 
 /**
  * Standard JdbcValuesMappingProducerProvider implementation
@@ -28,10 +34,20 @@ public class JdbcValuesMappingProducerProviderStandard implements JdbcValuesMapp
 	public JdbcValuesMappingProducer buildMappingProducer(
 			SelectStatement sqlAst,
 			SessionFactoryImplementor sessionFactory) {
-		return new JdbcValuesMappingProducerStandard(
-				sqlAst.getQuerySpec().getSelectClause().getSqlSelections(),
-				sqlAst.getDomainResultDescriptors()
-		);
+		return new JdbcValuesMappingProducerStandard( getSelections( sqlAst ), sqlAst.getDomainResultDescriptors() );
+	}
+
+	private static List<SqlSelection> getSelections(SelectStatement selectStatement) {
+		if ( selectStatement.getQueryPart() instanceof QueryGroup ) {
+			final QueryGroup queryGroup = (QueryGroup) selectStatement.getQueryPart();
+			for ( QueryPart queryPart : queryGroup.getQueryParts() ) {
+				if ( !(queryPart.getFirstQuerySpec().getSelectClause().getSqlSelections()
+						.get( 0 ).getExpressionType().getSingleJdbcMapping().getJdbcType() instanceof NullJdbcType) ) {
+					return queryPart.getFirstQuerySpec().getSelectClause().getSqlSelections();
+				}
+			}
+		}
+		return selectStatement.getQuerySpec().getSelectClause().getSqlSelections();
 	}
 
 	@Override
diff --git a/hibernate-core/src/test/java/org/hibernate/orm/test/query/hql/UnionAllSelectNullTest.java b/hibernate-core/src/test/java/org/hibernate/orm/test/query/hql/UnionAllSelectNullTest.java
new file mode 100644
index 000000000000..519be3f4c403
--- /dev/null
+++ b/hibernate-core/src/test/java/org/hibernate/orm/test/query/hql/UnionAllSelectNullTest.java
@@ -0,0 +1,140 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.orm.test.query.hql;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import jakarta.persistence.Tuple;
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.JiraKey;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+@DomainModel(
+		annotatedClasses = {
+				UnionAllSelectNullTest.TestEntity.class,
+				UnionAllSelectNullTest.AnotherTestEntity.class
+		}
+)
+@SessionFactory
+@JiraKey("HHH-18720")
+class UnionAllSelectNullTest {
+
+	@BeforeEach
+	public void setUp(SessionFactoryScope scope) {
+		scope.inTransaction(
+				session -> {
+					session.persist( new TestEntity( 1L, "a" ) );
+					session.persist( new AnotherTestEntity( 2L, "b" ) );
+				}
+		);
+	}
+
+	@AfterEach
+	public void tearDown(SessionFactoryScope scope) {
+		scope.inTransaction(
+				session -> {
+					session.createQuery( "delete TestEntity" ).executeUpdate();
+					session.createQuery( "delete AnotherTestEntity" ).executeUpdate();
+				}
+		);
+	}
+
+	@Test
+	void testSelect(SessionFactoryScope scope) {
+		scope.inTransaction(
+				session -> {
+					List<Tuple> resultList = session.createQuery(
+									"SELECT te.id as id from TestEntity te"
+											+ " union all SELECT null as id from AnotherTestEntity ate"
+									, Tuple.class )
+							.getResultList();
+					assertThat( resultList.size() ).isEqualTo( 2 );
+					assertResultIsCorrect( resultList, 1L, null );
+				}
+		);
+	}
+
+	@Test
+	void testSelect2(SessionFactoryScope scope) {
+		scope.inTransaction(
+				session -> {
+					List<Tuple> resultList = session.createQuery(
+									"SELECT null as id from TestEntity te"
+											+ " union all SELECT ate.id as id from AnotherTestEntity ate"
+									, Tuple.class )
+							.getResultList();
+					assertThat( resultList.size() ).isEqualTo( 2 );
+					assertResultIsCorrect( resultList, null, 2L );
+				}
+		);
+	}
+
+	@Test
+	void testSelect3(SessionFactoryScope scope) {
+		scope.inTransaction(
+				session -> {
+					List<Tuple> resultList = session.createQuery(
+									"SELECT te.id as id from TestEntity te"
+											+ " union all SELECT ate.id as id from AnotherTestEntity ate"
+									, Tuple.class )
+							.getResultList();
+					assertThat( resultList.size() ).isEqualTo( 2 );
+					assertResultIsCorrect( resultList, 1L, 2L );
+				}
+		);
+	}
+
+	private static void assertResultIsCorrect(List<Tuple> resultList, Long id1, Long id2) {
+		Set<Long> ids = new HashSet<>( 2 );
+		ids.add( (Long) resultList.get( 0 ).get( "id" ) );
+		ids.add( (Long) resultList.get( 1 ).get( "id" ) );
+		assertThat( ids.contains( id1 ) ).as( "Result does not contain expected value:" + id1 ).isTrue();
+		assertThat( ids.contains( id2 ) ).as( "Result does not contain expected value:" + id2 ).isTrue();
+	}
+
+	@Entity(name = "TestEntity")
+	public static class TestEntity {
+
+		@Id
+		private Long id;
+
+		private String name;
+
+		public TestEntity() {
+		}
+
+		public TestEntity(Long id, String name) {
+			this.id = id;
+			this.name = name;
+		}
+	}
+
+	@Entity(name = "AnotherTestEntity")
+	public static class AnotherTestEntity {
+
+		@Id
+		private Long id;
+
+		private String name;
+
+		public AnotherTestEntity() {
+		}
+
+		public AnotherTestEntity(Long id, String name) {
+			this.id = id;
+			this.name = name;
+		}
+	}
+}