diff --git a/data4k/src/main/kotlin/dev/forkhandles/data/DataContainer.kt b/data4k/src/main/kotlin/dev/forkhandles/data/DataContainer.kt
index 5e32cee..8ec17fe 100644
--- a/data4k/src/main/kotlin/dev/forkhandles/data/DataContainer.kt
+++ b/data4k/src/main/kotlin/dev/forkhandles/data/DataContainer.kt
@@ -22,7 +22,7 @@ abstract class DataContainer<CONTENT>(
         (get(it) as OUT)?.let(mapFn)
     }
 
-    fun <IN : Any, OUT : Value<IN>> field(factory: ValueFactory<OUT, IN>) = field(factory::of)
+    fun <IN : Any, OUT : Value<IN>, OUT2 : OUT?> field(factory: ValueFactory<OUT, IN>) = field(factory::of)
 
     fun <IN, OUT> list(mapFn: (IN) -> OUT) =
         DataProperty<DataContainer<CONTENT>, List<OUT>>(exists) { get(it)?.let { (it as List<IN>).map(mapFn) } }
@@ -31,7 +31,7 @@ abstract class DataContainer<CONTENT>(
 
     fun <OUT> list() = list<OUT, OUT> { it }
 
-    fun <OUT : DataContainer<CONTENT>> obj(mapFn: (CONTENT) -> OUT) =
+    fun <OUT : DataContainer<CONTENT>?> obj(mapFn: (CONTENT) -> OUT) =
         DataProperty<DataContainer<CONTENT>, OUT>(exists) { (get(it) as CONTENT)?.let(mapFn) }
 
     fun obj() = DataProperty<DataContainer<CONTENT>, CONTENT>(exists) { get(it) as CONTENT? }
diff --git a/data4k/src/test/kotlin/dev/forkhandles/lens/DataContainerContract.kt b/data4k/src/test/kotlin/dev/forkhandles/lens/DataContainerContract.kt
index ad6959f..c62fa52 100644
--- a/data4k/src/test/kotlin/dev/forkhandles/lens/DataContainerContract.kt
+++ b/data4k/src/test/kotlin/dev/forkhandles/lens/DataContainerContract.kt
@@ -75,6 +75,7 @@ abstract class DataContainerContract {
         expectThat(input.decimalField).isEqualTo(1.1234)
 
         expectThat(input.mappedField).isEqualTo(123)
+        expectThrows<ClassCastException> { container(mapOf("mappedField" to 123)).mappedField }
         expectThat(input.valueField).isEqualTo(MyType.of(123))
 
         expectThat(input.optionalField).isEqualTo("optional")
diff --git a/data4k/src/test/kotlin/dev/forkhandles/lens/MapDataContainerTest.kt b/data4k/src/test/kotlin/dev/forkhandles/lens/MapDataContainerTest.kt
index fee2421..6548404 100644
--- a/data4k/src/test/kotlin/dev/forkhandles/lens/MapDataContainerTest.kt
+++ b/data4k/src/test/kotlin/dev/forkhandles/lens/MapDataContainerTest.kt
@@ -30,7 +30,7 @@ class MapDataContainerTest : DataContainerContract() {
 
         override val optionalField by field<String?>()
         override val optionalListField: List<String>? by list()
-        override val optionalObjectField: SubClassFields? by obj(::SubMap)
+        override val optionalObjectField: SubMap? by obj(::SubMap)
         override val optionalValueField: MyType? by field(MyType)
     }