diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/AbstractSerialGenerator.kt b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/AbstractSerialGenerator.kt
index dda81c7cc1a52..d3d5c2a03030f 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/AbstractSerialGenerator.kt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/AbstractSerialGenerator.kt
@@ -46,11 +46,11 @@ abstract class AbstractSerialGenerator(val bindingContext: BindingContext, val c
         ).toSet()
     }
 
-    val additionalSerializersInScopeOfCurrentFile: Map<ClassDescriptor, ClassDescriptor> by lazy {
+    val additionalSerializersInScopeOfCurrentFile: Map<KotlinType, ClassDescriptor> by lazy {
         getKClassListFromFileAnnotation(SerializationAnnotations.additionalSerializersFqName, currentDeclaration)
             .associateBy(
                 {
-                    it.supertypes().find(::isKSerializer)?.arguments?.firstOrNull()?.type.toClassDescriptor
+                    it.supertypes().find(::isKSerializer)?.arguments?.firstOrNull()?.type
                         ?: throw AssertionError("Argument for ${SerializationAnnotations.additionalSerializersFqName} does not implement KSerializer or does not provide serializer for concrete type")
                 },
                 { it.toClassDescriptor!! }
@@ -59,4 +59,4 @@ abstract class AbstractSerialGenerator(val bindingContext: BindingContext, val c
 
     protected fun ClassDescriptor.getFuncDesc(funcName: String): Sequence<FunctionDescriptor> =
         unsubstitutedMemberScope.getDescriptorsFiltered { it == Name.identifier(funcName) }.asSequence().filterIsInstance<FunctionDescriptor>()
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt
index eab539e54a3d3..1fcb94c05d05e 100644
--- a/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt
+++ b/plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt
@@ -34,7 +34,7 @@ open class SerialTypeInfo(
 )
 
 fun AbstractSerialGenerator.findAddOnSerializer(propertyType: KotlinType, module: ModuleDescriptor): ClassDescriptor? {
-    additionalSerializersInScopeOfCurrentFile[propertyType.toClassDescriptor]?.let { return it }
+    additionalSerializersInScopeOfCurrentFile[propertyType]?.let { return it }
     if (propertyType in contextualKClassListInCurrentFile)
         return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     if (propertyType.toClassDescriptor?.annotations?.hasAnnotation(SerializationAnnotations.polymorphicFqName) == true)
@@ -123,9 +123,12 @@ fun AbstractSerialGenerator.findTypeSerializerOrContextUnchecked(
 ): ClassDescriptor? {
     val annotations = kType.annotations
     if (kType.isTypeParameter()) return null
-    if (kType.isMarkedNullable) return findTypeSerializerOrContextUnchecked(module, kType.makeNotNullable())
     annotations.serializableWith(module)?.let { return it.toClassDescriptor }
-    additionalSerializersInScopeOfCurrentFile[kType.toClassDescriptor]?.let { return it }
+    additionalSerializersInScopeOfCurrentFile[kType]?.let { return it }
+    if (!kType.isMarkedNullable) {
+        additionalSerializersInScopeOfCurrentFile[kType.makeNullable()]?.let { return it }
+    }
+
     if (kType in contextualKClassListInCurrentFile) return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     return analyzeSpecialSerializers(module, annotations) ?: findTypeSerializer(module, kType)
 }