Data Class with Nullable Parameter in Kotlin 1.5.0 & 1.6.0 Throwing BackendException

Recently updated from Kotlin 1.4.20 to 1.5.30.

I have this class which used to compile no problem

data class Optional<M>(val value: M?)

However, after upgrading, I get the following exception

org.jetbrains.kotlin.backend.common.BackendException: Backend Internal error: Exception during IR lowering
File being compiled: .../Optional.kt
The root cause java.lang.RuntimeException was thrown at: org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:50)
    at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException(CodegenUtil.kt:239)
    at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException$default(CodegenUtil.kt:235)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invokeSequential(performByIrFile.kt:68)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:55)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:41)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.invoke(CompilerPhase.kt:96)
    at org.jetbrains.kotlin.backend.common.phaser.CompositePhase.invoke(PhaseBuilders.kt:29)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.invoke(CompilerPhase.kt:96)
    at org.jetbrains.kotlin.backend.common.phaser.CompositePhase.invoke(PhaseBuilders.kt:29)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.invoke(CompilerPhase.kt:96)
    at org.jetbrains.kotlin.backend.common.phaser.CompilerPhaseKt.invokeToplevel(CompilerPhase.kt:43)
    at org.jetbrains.kotlin.backend.jvm.JvmIrCodegenFactory.doGenerateFilesInternal(JvmIrCodegenFactory.kt:191)
    at org.jetbrains.kotlin.backend.jvm.JvmIrCodegenFactory.generateModule(JvmIrCodegenFactory.kt:60)
    at org.jetbrains.kotlin.codegen.KotlinCodegenFacade.compileCorrectFiles(KotlinCodegenFacade.java:35)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.generate(KotlinToJVMBytecodeCompiler.kt:321)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli(KotlinToJVMBytecodeCompiler.kt:113)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli$default(KotlinToJVMBytecodeCompiler.kt:56)
    at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:169)
    at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:52)
    at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:92)
    at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:44)
    at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:98)
    at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:412)
    at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:112)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileIncrementally(IncrementalCompilerRunner.kt:358)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileIncrementally$default(IncrementalCompilerRunner.kt:300)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileImpl$rebuild(IncrementalCompilerRunner.kt:119)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileImpl(IncrementalCompilerRunner.kt:170)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compile(IncrementalCompilerRunner.kt:81)
    at org.jetbrains.kotlin.daemon.CompileServiceImplBase.execIncrementalCompiler(CompileServiceImpl.kt:607)
    at org.jetbrains.kotlin.daemon.CompileServiceImplBase.access$execIncrementalCompiler(CompileServiceImpl.kt:96)
    at org.jetbrains.kotlin.daemon.CompileServiceImpl.compile(CompileServiceImpl.kt:1658)

I get that it's telling me that Any's toString() method doesn't have a body as I can see that from looking at the source code.

What I don't understand is how to rectify it.

EDIT

Can confirm this still happens when upgrading to kotlin version 1.6.0.

It also happens in this data class:

data class Advertisement(
    var id: String,
    var image: String,
    var navUrl: String?
)

My build.gradle file for this module looks like so:

apply plugin: 'kotlin'
apply plugin: 'kotlin-kapt'
apply plugin: 'com.stustirling.redacted-gradle-plugin'

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"
    implementation "io.reactivex.rxjava2:rxkotlin:$rxKotlinVersion"
    implementation "io.reactivex.rxjava2:rxjava:$rxJavaVersion"
    implementation "com.jakewharton.timber:timber:$timberVersion"
    implementation "com.squareup.retrofit2:retrofit:$retrofitVersion"
    implementation "com.google.dagger:dagger:$daggerVersion"
    kapt "com.google.dagger:dagger-compiler:$daggerVersion"
    implementation "org.json:json:$testOrgJsonVersion"

    implementation "net.sf.biweekly:biweekly:$biweeklyVersion"

    testImplementation "junit:junit:$junitVersion"
    testImplementation "org.mockito.kotlin:mockito-kotlin:$mockitoKotlinVersion"
    testImplementation "org.mockito:mockito-inline:$mockitoInlineVersion"
    testImplementation "org.json:json:$testOrgJsonVersion"
}

redacted {
    redactAllDataClasses true
    redactClassName true
}


buildscript {
    repositories {
        mavenLocal()
        mavenCentral()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
    }
}