diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt
index 8f1dc121b91..766b851f03d 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt
@@ -404,7 +404,6 @@ open class SerializerIrGenerator(val irClass: IrClass, final override val compil
                         genericIndex = property.genericIndex
                     )
                     val isSerializable = innerSerial != null
-                    // todo: update
                     val decodeFuncToCall =
                         (if (isSerializable) "${CallingConventions.decode}${sti.elementMethodPrefix}Serializable${CallingConventions.elementPostfix}"
                         else "${CallingConventions.decode}${sti.elementMethodPrefix}${CallingConventions.elementPostfix}")
@@ -414,8 +413,8 @@ open class SerializerIrGenerator(val irClass: IrClass, final override val compil
                     val typeArgs =
                         if (decodeFuncToCall.descriptor.typeParameters.isNotEmpty()) listOf(property.type.toIrType()) else listOf()
                     val args = mutableListOf<IrExpression>(localSerialDesc.get(), irInt(index))
-                    if (innerSerial != null) {
-                        args.add(innerSerial)
+                    if (isSerializable) {
+                        args.add(innerSerial!!)
                         args.add(localProps[index].get())
                     }
                     // local$i = localInput.decode...(...)
diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializerCodegenImpl.kt b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializerCodegenImpl.kt
index 0d15f075d5b..a33358dfa79 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializerCodegenImpl.kt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializerCodegenImpl.kt
@@ -334,7 +334,7 @@ open class SerializerCodegenImpl(
             propVar = propsStartVar
             for ((index, property) in serializableProperties.withIndex()) {
                 val propertyType = codegen.typeMapper.mapType(property.type)
-                callReadProperty(property, propertyType, index, inputVar, descVar, -1, propVar)
+                callReadProperty(property, propertyType, index, inputVar, descVar, propVar)
                 propVar += propertyType.size
             }
             // set all bit masks to true
@@ -373,7 +373,7 @@ open class SerializerCodegenImpl(
                     val propertyAddressInBitMask = bitMaskOff(index)
                     // labelI:
                     visitLabel(labels[labelNum + 1])
-                    callReadProperty(property, propertyType, index, inputVar, descVar, propertyAddressInBitMask, propVar)
+                    callReadProperty(property, propertyType, index, inputVar, descVar, propVar)
 
                     // mark read bit in mask
                     // bitMask = bitMask | 1 << index
@@ -454,7 +454,6 @@ open class SerializerCodegenImpl(
         index: Int,
         inputVar: Int,
         descriptorVar: Int,
-        propertyAddressInBitMask: Int,
         propertyVar: Int
     ) {
         // propX := input.readXxxValue(value)
@@ -482,12 +481,12 @@ open class SerializerCodegenImpl(
             )
         }
 
-        val isUpdatable = useSerializer && propertyAddressInBitMask != -1
-        if (isUpdatable) {
+        if (useSerializer) {
+            // then it is not a primitive and can be updated via `oldValue` parameter in decodeSerializableElement
             load(propertyVar, propertyType)
             StackValue.coerce(propertyType, sti.type, this)
         }
-        produceCall(isUpdatable)
+        produceCall(useSerializer)
 
         StackValue.coerce(sti.type, propertyType, this)
         store(propertyVar, propertyType)
diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.txt b/plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.txt
index a821c67dbb6..da0e96240fb 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.txt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.txt
@@ -80,7 +80,8 @@ public final class ListOfUsers$$serializer : java/lang/Object, kotlinx/serializa
           GETSTATIC (INSTANCE, LUser$$serializer;)
           CHECKCAST
           INVOKESPECIAL (kotlinx/serialization/internal/ArrayListSerializer, <init>, (Lkotlinx/serialization/KSerializer;)V)
-          INVOKEINTERFACE (kotlinx/serialization/encoding/CompositeDecoder, decodeSerializableElement, (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;)Ljava/lang/Object;)
+          ALOAD (5)
+          INVOKEINTERFACE (kotlinx/serialization/encoding/CompositeDecoder, decodeSerializableElement, (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;Ljava/lang/Object;)Ljava/lang/Object;)
           CHECKCAST
           ASTORE (5)
           LDC (2147483647)
@@ -365,7 +366,8 @@ public final class OptionalUser$$serializer : java/lang/Object, kotlinx/serializ
           ICONST_0
           GETSTATIC (INSTANCE, LUser$$serializer;)
           CHECKCAST
-          INVOKEINTERFACE (kotlinx/serialization/encoding/CompositeDecoder, decodeSerializableElement, (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;)Ljava/lang/Object;)
+          ALOAD (5)
+          INVOKEINTERFACE (kotlinx/serialization/encoding/CompositeDecoder, decodeSerializableElement, (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;Ljava/lang/Object;)Ljava/lang/Object;)
           CHECKCAST
           ASTORE (5)
           LDC (2147483647)