diff --git a/spring-web/src/main/java/org/springframework/http/converter/AbstractHttpMessageConverter.java b/spring-web/src/main/java/org/springframework/http/converter/AbstractHttpMessageConverter.java
index 7b873b97b0..cf0674e06a 100644
--- a/spring-web/src/main/java/org/springframework/http/converter/AbstractHttpMessageConverter.java
+++ b/spring-web/src/main/java/org/springframework/http/converter/AbstractHttpMessageConverter.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -239,7 +239,7 @@ public abstract class AbstractHttpMessageConverter<T> implements HttpMessageConv
 	protected void addDefaultHeaders(HttpHeaders headers, T t, @Nullable MediaType contentType) throws IOException {
 		if (headers.getContentType() == null) {
 			MediaType contentTypeToUse = contentType;
-			if (contentType == null || contentType.isWildcardType() || contentType.isWildcardSubtype()) {
+			if (contentType == null || !contentType.isConcrete()) {
 				contentTypeToUse = getDefaultContentType(t);
 			}
 			else if (MediaType.APPLICATION_OCTET_STREAM.equals(contentType)) {
diff --git a/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java b/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
index 7abccf5a06..d576855d95 100644
--- a/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
+++ b/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -102,14 +102,14 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 
 
 	@Override
-	protected void addDefaultHeaders(HttpHeaders headers, String s, @Nullable MediaType mediaType) throws IOException {
+	protected void addDefaultHeaders(HttpHeaders headers, String s, @Nullable MediaType type) throws IOException {
 		if (headers.getContentType() == null ) {
-			if (mediaType != null && mediaType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
+			if (type != null && type.isConcrete() && type.isCompatibleWith(MediaType.APPLICATION_JSON)) {
 				// Prevent charset parameter for JSON..
-				headers.setContentType(mediaType);
+				headers.setContentType(type);
 			}
 		}
-		super.addDefaultHeaders(headers, s, mediaType);
+		super.addDefaultHeaders(headers, s, type);
 	}
 
 	@Override
diff --git a/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java b/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
index c0e8792d33..5ee6df9b00 100644
--- a/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
+++ b/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -131,4 +131,16 @@ public class StringHttpMessageConverterTests {
 		assertThat(headers.getAcceptCharset().isEmpty()).isTrue();
 	}
 
+	@Test // gh-24283
+	public void writeWithWildCardMediaType() throws IOException {
+		String body = "Hello World";
+		this.converter.write(body, MediaType.ALL, this.outputMessage);
+
+		HttpHeaders headers = this.outputMessage.getHeaders();
+		assertThat(this.outputMessage.getBodyAsString(StandardCharsets.US_ASCII)).isEqualTo(body);
+		assertThat(headers.getContentType()).isEqualTo(new MediaType("text", "plain", StandardCharsets.ISO_8859_1));
+		assertThat(headers.getContentLength()).isEqualTo(body.getBytes().length);
+		assertThat(headers.getAcceptCharset().isEmpty()).isTrue();
+	}
+
 }