diff --git a/bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt b/bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt
index 412a0483231..5431a765d48 100644
--- a/bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt
+++ b/bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt
@@ -18,10 +18,10 @@ package org.bson.codecs.kotlin
 import java.lang.reflect.ParameterizedType
 import java.lang.reflect.Type
 import kotlin.reflect.KClass
+import kotlin.reflect.KClassifier
 import kotlin.reflect.KFunction
 import kotlin.reflect.KParameter
 import kotlin.reflect.KProperty1
-import kotlin.reflect.KType
 import kotlin.reflect.KTypeParameter
 import kotlin.reflect.KTypeProjection
 import kotlin.reflect.full.createType
@@ -142,7 +142,9 @@ internal data class DataClassCodec<T : Any>(
                 val primaryConstructor =
                     kClass.primaryConstructor ?: throw CodecConfigurationException("No primary constructor for $kClass")
                 val typeMap =
-                    types.mapIndexed { i, k -> primaryConstructor.typeParameters[i].createType() to k }.toMap()
+                    types
+                        .mapIndexed { i, k -> primaryConstructor.typeParameters[i].createType().classifier!! to k }
+                        .toMap()
 
                 val propertyModels =
                     primaryConstructor.parameters.map { kParameter ->
@@ -193,7 +195,7 @@ internal data class DataClassCodec<T : Any>(
         @Suppress("UNCHECKED_CAST")
         private fun getCodec(
             kParameter: KParameter,
-            typeMap: Map<KType, Type>,
+            typeMap: Map<KClassifier, Type>,
             codecRegistry: CodecRegistry
         ): Codec<Any> {
             return when (kParameter.type.classifier) {
@@ -201,10 +203,12 @@ internal data class DataClassCodec<T : Any>(
                     codecRegistry.getCodec(
                         kParameter,
                         (kParameter.type.classifier as KClass<Any>).javaObjectType,
-                        kParameter.type.arguments.mapNotNull { typeMap[it.type] ?: computeJavaType(it) }.toList())
+                        kParameter.type.arguments
+                            .mapNotNull { typeMap[it.type?.classifier] ?: computeJavaType(it) }
+                            .toList())
                 }
                 is KTypeParameter -> {
-                    when (val pType = typeMap[kParameter.type] ?: kParameter.type.javaType) {
+                    when (val pType = typeMap[kParameter.type.classifier] ?: kParameter.type.javaType) {
                         is Class<*> ->
                             codecRegistry.getCodec(kParameter, (pType as Class<Any>).kotlin.javaObjectType, emptyList())
                         is ParameterizedType ->
diff --git a/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt b/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt
index d2dbfc580cc..40abc3a9cfa 100644
--- a/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt
+++ b/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt
@@ -249,7 +249,7 @@ class DataClassCodecTest {
             |"nestedParameterized": {
             |  "parameterizedDataClass":
             |  {"number": 4.2, "string": "myString", "parameterizedList": [{"name": "embedded1"}]},
-            |  "other": "myOtherString"
+            |  "other": "myOtherString", "optionalOther": "myOptionalOtherString"
             | }
             |}"""
                 .trimMargin()
@@ -257,7 +257,9 @@ class DataClassCodecTest {
             DataClassWithNestedParameterizedDataClass(
                 "myId",
                 DataClassWithNestedParameterized(
-                    DataClassParameterized(4.2, "myString", listOf(DataClassEmbedded("embedded1"))), "myOtherString"))
+                    DataClassParameterized(4.2, "myString", listOf(DataClassEmbedded("embedded1"))),
+                    "myOtherString",
+                    "myOptionalOtherString"))
 
         assertRoundTrips(expected, dataClass)
     }
diff --git a/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/samples/DataClasses.kt b/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/samples/DataClasses.kt
index a320470cf23..5bc6e768ed8 100644
--- a/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/samples/DataClasses.kt
+++ b/bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/samples/DataClasses.kt
@@ -89,7 +89,8 @@ data class DataClassWithNestedParameterizedDataClass(
 
 data class DataClassWithNestedParameterized<A, B, C : Number>(
     val parameterizedDataClass: DataClassParameterized<C, A>,
-    val other: B
+    val other: B,
+    val optionalOther: B?
 )
 
 data class DataClassWithPair(val pair: Pair<String, Int>)