"Two different classes used for type" reports null class

[also]

With graphql-java-tools 3.1.2, I get an error that refers to the class null instead of the actual class:

Exception in thread "main" com.coxautodev.graphql.tools.SchemaClassScannerError: Two different classes used for type Scalar:
- class java.lang.Object:
|   return type of method public java.lang.Object com.example.GraphQLToolsErrorExample$Query.a()

- null:
|   return type of method public java.lang.String com.example.GraphQLToolsErrorExample$Query.b()
	at com.coxautodev.graphql.tools.SchemaClassScanner.handleFoundType(SchemaClassScanner.kt:208)
	at com.coxautodev.graphql.tools.SchemaClassScanner.handleFoundType(SchemaClassScanner.kt:193)
	at com.coxautodev.graphql.tools.SchemaClassScanner.handleFieldMethod(SchemaClassScanner.kt:185)
	at com.coxautodev.graphql.tools.SchemaClassScanner.scanObjectForDictionaryItems(SchemaClassScanner.kt:177)
	at com.coxautodev.graphql.tools.SchemaClassScanner.scanForClasses(SchemaClassScanner.kt:95)
	at com.coxautodev.graphql.tools.SchemaParserBuilder.build(SchemaParserBuilder.kt:107)
	at com.example.GraphQLToolsErrorExample.main(GraphQLToolsErrorExample.java:37)

Here's a simple example to reproduce:

package com.example;

import com.coxautodev.graphql.tools.GraphQLRootResolver;
import com.coxautodev.graphql.tools.SchemaParser;

public class GraphQLToolsErrorExample {
  public static class Query implements GraphQLRootResolver {
    public Object a() {
      return "a";
    }
    public String b() {
      return "b";
    }
  }
  public static void main(String... args) {
    SchemaParser.newParser()
        .schemaString("scalar Scalar type Query { a: Scalar b: Scalar}")
        .resolvers(new Query())
        .build()
        .makeExecutableSchema();
  }
}