diff --git a/compiler/src/main/kotlin/se/ansman/kotshi/kapt/generators/ObjectAdapterGenerator.kt b/compiler/src/main/kotlin/se/ansman/kotshi/kapt/generators/ObjectAdapterGenerator.kt
index 5dd29b8..053a632 100644
--- a/compiler/src/main/kotlin/se/ansman/kotshi/kapt/generators/ObjectAdapterGenerator.kt
+++ b/compiler/src/main/kotlin/se/ansman/kotshi/kapt/generators/ObjectAdapterGenerator.kt
@@ -5,7 +5,7 @@ import kotlinx.metadata.isData
 import se.ansman.kotshi.Errors
 import se.ansman.kotshi.kapt.MetadataAccessor
 import se.ansman.kotshi.kapt.isObject
-import se.ansman.kotshi.kapt.logKotshiWarning
+import se.ansman.kotshi.kapt.logKotshiError
 import se.ansman.kotshi.model.GeneratableJsonAdapter
 import se.ansman.kotshi.model.GlobalConfig
 import se.ansman.kotshi.model.ObjectJsonAdapter
@@ -26,7 +26,7 @@ class ObjectAdapterGenerator(
     init {
         require(metadata.isObject)
         if (!metadata.isData && metadataAccessor.getLanguageVersion(element) >= KotlinVersion(1, 9)) {
-            messager.logKotshiWarning(Errors.nonDataObject, element)
+            messager.logKotshiError(Errors.nonDataObject, element)
         }
     }
 
diff --git a/compiler/src/main/kotlin/se/ansman/kotshi/ksp/generators/ObjectAdapterGenerator.kt b/compiler/src/main/kotlin/se/ansman/kotshi/ksp/generators/ObjectAdapterGenerator.kt
index 0f69714..ba0be74 100644
--- a/compiler/src/main/kotlin/se/ansman/kotshi/ksp/generators/ObjectAdapterGenerator.kt
+++ b/compiler/src/main/kotlin/se/ansman/kotshi/ksp/generators/ObjectAdapterGenerator.kt
@@ -6,7 +6,7 @@ import com.google.devtools.ksp.symbol.ClassKind
 import com.google.devtools.ksp.symbol.KSClassDeclaration
 import com.google.devtools.ksp.symbol.Modifier
 import se.ansman.kotshi.Errors
-import se.ansman.kotshi.ksp.logKotshiWarning
+import se.ansman.kotshi.ksp.logKotshiError
 import se.ansman.kotshi.model.GeneratableJsonAdapter
 import se.ansman.kotshi.model.GlobalConfig
 import se.ansman.kotshi.model.ObjectJsonAdapter
@@ -20,7 +20,7 @@ class ObjectAdapterGenerator(
     init {
         require(element.classKind == ClassKind.OBJECT)
         if (Modifier.DATA !in element.modifiers && environment.apiVersion >= KotlinVersion(1, 9)) {
-            environment.logger.logKotshiWarning(Errors.nonDataObject, element)
+            environment.logger.logKotshiError(Errors.nonDataObject, element)
         }
     }
 
diff --git a/tests/ksp/src/test/kotlin/se/ansman/kotshi/KspGeneratorTest.kt b/tests/ksp/src/test/kotlin/se/ansman/kotshi/KspGeneratorTest.kt
index c58c5ae..799b45c 100644
--- a/tests/ksp/src/test/kotlin/se/ansman/kotshi/KspGeneratorTest.kt
+++ b/tests/ksp/src/test/kotlin/se/ansman/kotshi/KspGeneratorTest.kt
@@ -227,7 +227,7 @@ class KspGeneratorTest : BaseGeneratorTest() {
         object2 = SourceFile.kotlin(
             "object2.kt", """
             @se.ansman.kotshi.JsonSerializable
-            object Object2
+            data object Object2
         """.trimIndent()
         )
         with(compile(factory, object1, object2)) {
@@ -318,7 +318,4 @@ class KspGeneratorTest : BaseGeneratorTest() {
             )
         }
     }
-
-    @Disabled("kotlin-compile-testing doesn't work well with overriding language version yet")
-    override fun `non data object logs warnings`() {}
 }
\ No newline at end of file
diff --git a/tests/src/test/kotlin/se/ansman/kotshi/BaseGeneratorTest.kt b/tests/src/test/kotlin/se/ansman/kotshi/BaseGeneratorTest.kt
index e04ad00..95984bd 100644
--- a/tests/src/test/kotlin/se/ansman/kotshi/BaseGeneratorTest.kt
+++ b/tests/src/test/kotlin/se/ansman/kotshi/BaseGeneratorTest.kt
@@ -574,13 +574,13 @@ abstract class BaseGeneratorTest {
     }
 
     @Test
-    open fun `non data object logs warnings`() {
+    fun `non data object fails the build`() {
         val source = kotlin("source.kt", """
             @se.ansman.kotshi.JsonSerializable
             object TestObject
         """)
         val result = compile(source)
-        assertThat(result::exitCode).isEqualTo(KotlinCompilation.ExitCode.OK)
+        assertThat(result::exitCode).isEqualTo(KotlinCompilation.ExitCode.COMPILATION_ERROR)
         assertThat(result::messages).contains(Errors.nonDataObject)
     }
 
@@ -619,7 +619,6 @@ abstract class BaseGeneratorTest {
                 workingDir = temporaryFolder
                 this.sources = sources.asList()
                 inheritClassPath = true
-                // TODO: Remove once kotlin-compile-testing supports Kotlin 1.9
                 this.languageVersion = languageVersion
                 messageOutputStream = System.out // see diagnostics in real time
                 setUp(options)