AWS Lambda handler written in Java 17 fails with a JSON error

Question

I've upgraded my environment to Java 17 and my AWS Lambda handler is now failing with a mystery error. The handler:

public class LambdaHandler implements RequestHandler<ScheduledEvent, String> {

  @Override public String handleRequest(ScheduledEvent event, Context context) {
    return "AOK";
  }

The pom.xml Maven build file:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>urisman</groupId>
    <artifactId>wp.monitor</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>Monitor Wordpress sites with AWS Lambda</name>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>com.amazonaws</groupId>
            <artifactId>aws-lambda-java-core</artifactId>
            <version>1.2.1</version>
        </dependency>
        <dependency>
            <groupId>com.amazonaws</groupId>
            <artifactId>aws-lambda-java-events</artifactId>
            <version>3.11.5</version>
        </dependency>
        <dependency>
            <groupId>com.amazonaws</groupId>
            <artifactId>aws-java-sdk-ses</artifactId>
            <version>1.12.732</version>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>5.8.2</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>5.8.2</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.22.2</version>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.2.2</version>
                <configuration>
                    <createDependencyReducedPom>false</createDependencyReducedPom>
                    <filters>
                        <filter>
                            <artifact>*:*</artifact>
                            <excludes>
                                <exclude>module-info.class</exclude>
                                <exclude>META-INF/*</exclude>
                                <exclude>META-INF/versions/**</exclude>
                                <exclude>META-INF/services/**</exclude>
                            </excludes>
                        </filter>
                    </filters>
                </configuration>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>17</source>
                    <target>17</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

This is a straight cut'n'paste from the AWS example app, sans the logging library which I don't use.

I am using ScheduledEvent because the lambda is invoked by a scheduler in EventBridge. The invocation produces the following bizarre error:

An error occurred during JSON parsing: java.lang.RuntimeException
java.lang.RuntimeException: An error occurred during JSON parsing
Caused by: java.io.UncheckedIOException: com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `java.util.LinkedHashMap` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('
{}
')
 at [Source: (ByteArrayInputStream); line: 1, column: 296] (through reference chain: com.amazonaws.services.lambda.runtime.events.ScheduledEvent["detail"])
    at com.amazonaws.services.lambda.runtime.serialization.factories.JacksonFactory$InternalSerializer.fromJson(JacksonFactory.java:176)
Caused by: com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `java.util.LinkedHashMap` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('
{}
')
 at [Source: (ByteArrayInputStream); line: 1, column: 296] (through reference chain: com.amazonaws.services.lambda.runtime.events.ScheduledEvent["detail"])
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1733)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1358)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromString(StdDeserializer.java:311)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:454)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:32)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:129)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:314)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:177)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:323)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.ObjectReader._bindAndClose(ObjectReader.java:2105)
    at com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.ObjectReader.readValue(ObjectReader.java:1481)
    at com.amazonaws.services.lambda.runtime.serialization.factories.JacksonFactory$InternalSerializer.fromJson(JacksonFactory.java:174)

Many thanks in advance for any pointers.