diff --git a/hibernate-community-dialects/src/main/java/org/hibernate/community/dialect/InformixDialect.java b/hibernate-community-dialects/src/main/java/org/hibernate/community/dialect/InformixDialect.java
index 00743146139d..139b5eed73ca 100644
--- a/hibernate-community-dialects/src/main/java/org/hibernate/community/dialect/InformixDialect.java
+++ b/hibernate-community-dialects/src/main/java/org/hibernate/community/dialect/InformixDialect.java
@@ -40,6 +40,8 @@
 import org.hibernate.exception.LockAcquisitionException;
 import org.hibernate.exception.spi.SQLExceptionConversionDelegate;
 import org.hibernate.mapping.CheckConstraint;
+import org.hibernate.metamodel.mapping.SqlExpressible;
+import org.hibernate.metamodel.mapping.SqlTypedMapping;
 import org.hibernate.query.sqm.CastType;
 import org.hibernate.query.sqm.IntervalType;
 import org.hibernate.query.sqm.function.SqmFunctionRegistry;
@@ -1097,6 +1099,17 @@ public String getSelectClauseNullString(int sqlType, TypeConfiguration typeConfi
 		return "cast(null as " + castType + ")";
 	}
 
+	//	Add override for the newer signature to access columnDefinition
+	@Override
+	public String getSelectClauseNullString(SqlTypedMapping sqlType, TypeConfiguration typeConfiguration) {
+		final DdlTypeRegistry ddlTypeRegistry = typeConfiguration.getDdlTypeRegistry();
+		final String castTypeName = ddlTypeRegistry
+				.getDescriptor( sqlType.getJdbcMapping().getJdbcType().getDdlTypeCode() )
+				.getCastTypeName( sqlType.toSize(), (SqlExpressible) sqlType.getJdbcMapping(), ddlTypeRegistry );
+		return "cast(null as " + castTypeName + ")";
+	}
+
+
 	private static String castType(DdlType descriptor) {
 		final String typeName = descriptor.getTypeName( Size.length( Size.DEFAULT_LENGTH ) );
 		//trim off the length/precision/scale
diff --git a/hibernate-community-dialects/src/test/java/org/hibernate/community/dialect/InformixUnionNullCastingTest.java b/hibernate-community-dialects/src/test/java/org/hibernate/community/dialect/InformixUnionNullCastingTest.java
new file mode 100644
index 000000000000..dd53be1a4740
--- /dev/null
+++ b/hibernate-community-dialects/src/test/java/org/hibernate/community/dialect/InformixUnionNullCastingTest.java
@@ -0,0 +1,156 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.community.dialect;
+
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.Id;
+import jakarta.persistence.Inheritance;
+import jakarta.persistence.InheritanceType;
+import jakarta.persistence.Lob;
+import org.hibernate.annotations.JdbcTypeCode;
+import org.hibernate.cfg.AvailableSettings;
+import org.hibernate.testing.jdbc.SQLStatementInspector;
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.JiraKey;
+import org.hibernate.testing.orm.junit.RequiresDialect;
+import org.hibernate.testing.orm.junit.ServiceRegistry;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.hibernate.testing.orm.junit.Setting;
+import org.hibernate.type.SqlTypes;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+
+@RequiresDialect(org.hibernate.community.dialect.InformixDialect.class)
+@DomainModel(annotatedClasses = {
+		InformixUnionNullCastingTest.BaseEntity.class,
+		InformixUnionNullCastingTest.LvarcharEntity.class,
+		InformixUnionNullCastingTest.DatetimeEntity.class,
+		InformixUnionNullCastingTest.ClobEntity.class,
+		InformixUnionNullCastingTest.EmptyEntity.class
+})
+@SessionFactory(useCollectingStatementInspector = true)
+@ServiceRegistry(
+		settings = {
+				@Setting(
+						name = AvailableSettings.DIALECT,
+						value = "org.hibernate.community.dialect.InformixDialect"
+				),
+				@Setting(name = AvailableSettings.SHOW_SQL, value = "true"),
+				@Setting(name = AvailableSettings.FORMAT_SQL, value = "true"),
+		}
+)
+public class InformixUnionNullCastingTest {
+
+	@BeforeEach
+	protected void setupTest(SessionFactoryScope scope) {
+		scope.inTransaction(
+				(session) -> {
+					session.persist(new EmptyEntity());
+
+					LvarcharEntity lvarcharEntity = new LvarcharEntity();
+					lvarcharEntity.longContent = "This is a very long string designed to be stored as LVARCHAR in Informix.";
+					session.persist(lvarcharEntity);
+
+					ClobEntity clobEntity = new ClobEntity();
+					clobEntity.clobContent = "This is a CLOB content that should be cast correctly in union queries.";
+					session.persist(clobEntity);
+
+					DatetimeEntity datetimeEntity = new DatetimeEntity();
+					datetimeEntity.eventTime = LocalDateTime.now();
+					session.persist(datetimeEntity);
+				}
+		);
+	}
+
+	@Test
+	@JiraKey(value = "HHH-19974")
+	public void testInformixSpecialTypeCasting(SessionFactoryScope scope) {
+		scope.inTransaction(session ->
+				session.createQuery(
+						"select r from root_entity r",
+						BaseEntity.class
+				).list()
+		);
+
+		SQLStatementInspector inspector = scope.getCollectingStatementInspector();
+		List<String> sqlQueries = inspector.getSqlQueries();
+
+		sqlQueries.forEach(System.out::println);
+
+		Stream.of(
+				new CastExpectation("LVARCHAR", "cast(null as lvarchar)"),
+				new CastExpectation("DATETIME", "cast(null as datetime", "year to"),
+				new CastExpectation("CLOB", "cast(null as clob)")
+		).forEach(expectation -> verifyCastings(sqlQueries, expectation));
+	}
+
+
+	private void verifyCastings(List<String> sqlQueries, CastExpectation expectation) {
+		assertThat(sqlQueries)
+				.as("SQL should contain proper cast for type: %s", expectation.typeName)
+				.anyMatch(sql -> {
+					String lowerSql = sql.toLowerCase();
+					return expectation.requiredPatterns.stream()
+							.allMatch(lowerSql::contains);
+				});
+	}
+
+	/**
+	 * A simple record to hold expectation data for verification.
+	 *
+	 * @param typeName The name of the type, used for error messaging.
+	 * @param requiredPatterns The list of lowercase string patterns that must be present in the generated SQL.
+	 */
+	record CastExpectation(String typeName, List<String> requiredPatterns) {
+		CastExpectation(String typeName, String... patterns) {
+			this(typeName, List.of(patterns));
+		}
+	}
+
+
+	@Entity(name = "root_entity")
+	@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
+	public static abstract class BaseEntity {
+		@Id
+		@GeneratedValue
+		private Long id;
+	}
+
+	@Entity(name = "lvarchar_entity")
+	public static class LvarcharEntity extends BaseEntity {
+
+		@JdbcTypeCode(SqlTypes.LONGVARCHAR)
+		public String longContent;
+	}
+
+	@Entity(name = "datetime_entity")
+	public static class DatetimeEntity extends BaseEntity {
+
+		public LocalDateTime eventTime;
+	}
+
+	@Entity(name = "clob_entity")
+	public static class ClobEntity extends BaseEntity {
+
+		@Lob
+		@JdbcTypeCode(SqlTypes.CLOB)
+		public String clobContent;
+	}
+
+	@Entity(name = "empty_entity")
+	public static class EmptyEntity extends BaseEntity {
+	}
+
+}