diff --git a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
index 53b7b804b61..7c8d57d1159 100644
--- a/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
+++ b/spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java
@@ -132,24 +132,25 @@ public abstract class KotlinSerializationSupport<T extends SerialFormat> {
 			Assert.notNull(method, "Method must not be null");
 			if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
 				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-				Assert.notNull(function, "Kotlin function must not be null");
-				KType type = (parameter.getParameterIndex() == -1 ? function.getReturnType() :
-						KCallables.getValueParameters(function).get(parameter.getParameterIndex()).getType());
-				KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
-				if (serializer == null) {
-					try {
-						serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
-					}
-					catch (IllegalArgumentException ignored) {
-					}
-					if (serializer != null) {
-						if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-							return null;
+				if (function != null) {
+					KType type = (parameter.getParameterIndex() == -1 ? function.getReturnType() :
+							KCallables.getValueParameters(function).get(parameter.getParameterIndex()).getType());
+					KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
+					if (serializer == null) {
+						try {
+							serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
+						}
+						catch (IllegalArgumentException ignored) {
+						}
+						if (serializer != null) {
+							if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
+								return null;
+							}
+							this.kTypeSerializerCache.put(type, serializer);
 						}
-						this.kTypeSerializerCache.put(type, serializer);
 					}
+					return serializer;
 				}
-				return serializer;
 			}
 		}
 		Type type = resolvableType.getType();
diff --git a/spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.java b/spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.java
index a6f75e27b54..ca0c65695a3 100644
--- a/spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.java
+++ b/spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.java
@@ -149,24 +149,25 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 			Assert.notNull(method, "Method must not be null");
 			if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
 				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-				Assert.notNull(function, "Kotlin function must not be null");
-				KType type = (parameter.getParameterIndex() == -1 ? function.getReturnType() :
-						KCallables.getValueParameters(function).get(parameter.getParameterIndex()).getType());
-				KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
-				if (serializer == null) {
-					try {
-						serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
-					}
-					catch (IllegalArgumentException ignored) {
-					}
-					if (serializer != null) {
-						if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-							return null;
+				if (function != null) {
+					KType type = (parameter.getParameterIndex() == -1 ? function.getReturnType() :
+							KCallables.getValueParameters(function).get(parameter.getParameterIndex()).getType());
+					KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
+					if (serializer == null) {
+						try {
+							serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
+						}
+						catch (IllegalArgumentException ignored) {
+						}
+						if (serializer != null) {
+							if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
+								return null;
+							}
+							this.kTypeSerializerCache.put(type, serializer);
 						}
-						this.kTypeSerializerCache.put(type, serializer);
 					}
+					return serializer;
 				}
-				return serializer;
 			}
 		}
 		Type type = resolvableType.getType();
diff --git a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
index 86c89a0d160..d5c92147929 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt
@@ -145,10 +145,24 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 		assertThat(encoder.canEncode(ResolvableType.forClass(BigDecimal::class.java), null)).isFalse()
 	}
 
+	@Test
+	fun encodeProperty() {
+		val input = Mono.just(value)
+		val method = this::class.java.getDeclaredMethod("getValue")
+		val methodParameter = MethodParameter.forExecutable(method, -1)
+		testEncode(input, ResolvableType.forMethodParameter(methodParameter), null, null) {
+			it.consumeNextWith(expectString("42"))
+				.verifyComplete()
+		}
+	}
+
 
 	@Serializable
 	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
 	fun handleMapWithNullable(map: Map<String, String?>) = map
 
+	val value: Int
+		get() = 42
+
 }
diff --git a/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt b/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
index 4c3b9b5dd0b..a495d9136b5 100644
--- a/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
+++ b/spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,18 +16,11 @@
 
 package org.springframework.http.converter.json
 
-import java.lang.reflect.ParameterizedType
-import java.lang.reflect.Type
-import java.nio.charset.StandardCharsets
-
 import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatExceptionOfType
 import org.junit.jupiter.api.Test
 import org.springframework.core.MethodParameter
-import kotlin.reflect.javaType
-import kotlin.reflect.typeOf
-
 import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
 import org.springframework.http.MediaType
@@ -35,8 +28,12 @@ import org.springframework.http.converter.HttpMessageNotReadableException
 import org.springframework.http.customJson
 import org.springframework.web.testfixture.http.MockHttpInputMessage
 import org.springframework.web.testfixture.http.MockHttpOutputMessage
+import java.lang.reflect.ParameterizedType
 import java.math.BigDecimal
+import java.nio.charset.StandardCharsets
+import kotlin.reflect.javaType
 import kotlin.reflect.jvm.javaMethod
+import kotlin.reflect.typeOf
 
 /**
  * Tests for the JSON conversion using kotlinx.serialization.
@@ -395,6 +392,20 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(result).isEqualTo(expectedJson)
 	}
 
+	@Test
+	fun writeProperty() {
+		val outputMessage = MockHttpOutputMessage()
+		val method = this::class.java.getDeclaredMethod("getValue")
+		val methodParameter = MethodParameter.forExecutable(method, -1)
+
+		this.converter.write(value, ResolvableType.forMethodParameter(methodParameter), null, outputMessage, null)
+		val result = outputMessage.getBodyAsString(StandardCharsets.UTF_8)
+
+		@Suppress("DEPRECATION")
+		assertThat(outputMessage.headers.asMultiValueMap()).containsEntry("Content-Type", listOf("application/json"))
+		assertThat(result).isEqualTo("42")
+	}
+
 
 	@Serializable
 	@Suppress("ArrayInDataClass")
@@ -420,4 +431,7 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 
 	fun handleMapWithNullable(map: Map<String, String?>) = map
 
+	val value: Int
+		get() = 42
+
 }