diff --git a/spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java b/spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java
index 9286949c..128887b3 100644
--- a/spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java
+++ b/spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java
@@ -71,8 +71,10 @@ public class ModifyRequestBodyGatewayFilterFactory extends
 
 				// TODO: flux or mono
 				Mono<?> modifiedBody = serverRequest.bodyToMono(inClass)
-						// .log("modify_request_mono", Level.INFO)
-						.flatMap(o -> config.rewriteFunction.apply(exchange, o));
+						.flatMap(originalBody -> config.getRewriteFunction()
+								.apply(exchange, originalBody))
+						.switchIfEmpty(Mono.defer(() -> (Mono) config.getRewriteFunction()
+								.apply(exchange, null)));
 
 				BodyInserter bodyInserter = BodyInserters.fromPublisher(modifiedBody,
 						config.getOutClass());
diff --git a/spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactoryTests.java b/spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactoryTests.java
index c9331df8..f52a7fad 100644
--- a/spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactoryTests.java
+++ b/spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactoryTests.java
@@ -56,6 +56,17 @@ public class ModifyRequestBodyGatewayFilterFactoryTests extends BaseWebClientTes
 				.isEqualTo("modifyrequest");
 	}
 
+	@Test
+	public void upstreamRequestBodyIsEmpty() {
+		testClient.post().uri("/post").header("Host", "www.modifyrequestbodyempty.org")
+				.header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
+				.exchange().expectStatus().isEqualTo(HttpStatus.OK).expectBody()
+				.jsonPath("headers.Content-Type")
+				.isEqualTo(MediaType.APPLICATION_JSON_VALUE).jsonPath("data")
+				.isEqualTo("modifyrequest");
+	}
+
+
 	@EnableAutoConfiguration
 	@SpringBootConfiguration
 	@Import(DefaultTestConfig.class)
@@ -74,6 +85,17 @@ public class ModifyRequestBodyGatewayFilterFactoryTests extends BaseWebClientTes
 										return Mono.just("modifyrequest");
 									}))
 							.uri(uri))
+					.route("test_modify_request_body_empty",
+							r -> r.order(-1).host("**.modifyrequestbodyempty.org")
+									.filters(f -> f.modifyRequestBody(String.class, String.class,
+											MediaType.APPLICATION_JSON_VALUE,
+											(serverWebExchange, body) -> {
+												if (body == null) {
+													return Mono.just("modifyrequest");
+												}
+												return Mono.just(body.toUpperCase());
+											}))
+									.uri(uri))
 					.build();
 		}