diff --git a/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/TargetType.kt b/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/TargetType.kt
index 6d93fd9..305afd3 100644
--- a/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/TargetType.kt
+++ b/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/TargetType.kt
@@ -73,7 +73,7 @@ internal data class TargetType(
       when {
         proto.classKind == Class.Kind.ENUM_CLASS -> {
           messager.printMessage(
-              ERROR, "@JsonClass can't be applied to $element: must not be an enum class", element)
+              ERROR, "@JsonClass with 'generateAdapter = \"true\"' can't be applied to $element: code gen for enums is not supported or necessary", element)
           return null
         }
         proto.classKind != Class.Kind.CLASS -> {
diff --git a/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt b/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
index 2d66f21..dbaacce 100644
--- a/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
+++ b/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
@@ -153,7 +153,7 @@ class JsonClassCodegenProcessorTest {
     val result = call.execute()
     assertThat(result.exitCode).isEqualTo(ExitCode.COMPILATION_ERROR)
     assertThat(result.systemErr).contains(
-        "error: @JsonClass can't be applied to KotlinEnum: must not be an enum class")
+        "error: @JsonClass with 'generateAdapter = \"true\"' can't be applied to KotlinEnum: code gen for enums is not supported or necessary")
   }
 
   // Annotation processors don't get called for local classes, so we don't have the opportunity to