diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/GeneratorHelpers.kt b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/GeneratorHelpers.kt
index e175e9b1361..97c4ad2ef53 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/GeneratorHelpers.kt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/GeneratorHelpers.kt
@@ -1135,9 +1135,10 @@ interface IrBuilderExtension {
         } ?: return null
 
         val adjustedArgs: List<IrExpression> =
-            if (baseClass.descriptor.isSealed() || baseClass.descriptor.modality == Modality.ABSTRACT) {
+            // if typeArgs.size == args.size then the serializer is custom - we need to use the actual serializers from the arguments
+            if ((typeArgs.size != args.size) && (baseClass.descriptor.isSealed() || baseClass.descriptor.modality == Modality.ABSTRACT)) {
                 val serializer = findStandardKotlinTypeSerializer(baseClass.module, context.irBuiltIns.unitType.toKotlinType())!!
-                // workaround for sealed and classes - the `serializer` function expects non-null serializers, but does not use them, so serializers of any type can be passed
+                // workaround for sealed and abstract classes - the `serializer` function expects non-null serializers, but does not use them, so serializers of any type can be passed
                 List(baseClass.typeParameters.size) { irGetObject(serializer) }
             } else {
                 args