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 fd123b97a8e..b44546be349 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
@@ -38,12 +38,12 @@ abstract class AbstractSerialGenerator(val bindingContext: BindingContext, val c
         ).toSet()
     }
 
-    val additionalSerializersInScopeOfCurrentFile: Map<KotlinType, ClassDescriptor> by lazy {
+    val additionalSerializersInScopeOfCurrentFile: Map<ClassDescriptor, ClassDescriptor> by lazy {
         getKClassListFromFileAnnotation(SerializationAnnotations.additionalSerializersFqName, currentDeclaration)
             .associateBy(
                 {
-                    it.supertypes().find(::isKSerializer)?.arguments?.firstOrNull()?.type
-                        ?: throw AssertionError("Argument for ${SerializationAnnotations.additionalSerializersFqName} does not implement KSerializer")
+                    it.supertypes().find(::isKSerializer)?.arguments?.firstOrNull()?.type.toClassDescriptor
+                        ?: throw AssertionError("Argument for ${SerializationAnnotations.additionalSerializersFqName} does not implement KSerializer or does not provide serializer for concrete type")
                 },
                 { it.toClassDescriptor!! }
             )
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 cb19d4bef34..2fc7c073251 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
@@ -46,7 +46,7 @@ open class SerialTypeInfo(
 )
 
 fun AbstractSerialGenerator.findAddOnSerializer(propertyType: KotlinType, module: ModuleDescriptor): ClassDescriptor? {
-    additionalSerializersInScopeOfCurrentFile[propertyType]?.let { return it }
+    additionalSerializersInScopeOfCurrentFile[propertyType.toClassDescriptor]?.let { return it }
     if (propertyType in contextualKClassListInCurrentFile)
         return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     if (propertyType.toClassDescriptor?.annotations?.hasAnnotation(SerializationAnnotations.polymorphicFqName) == true)
@@ -136,7 +136,7 @@ fun AbstractSerialGenerator.findTypeSerializerOrContextUnchecked(
     if (kType.isTypeParameter()) return null
     if (kType.isMarkedNullable) return findTypeSerializerOrContextUnchecked(module, kType.makeNotNullable())
     annotations.serializableWith(module)?.let { return it.toClassDescriptor }
-    additionalSerializersInScopeOfCurrentFile[kType]?.let { return it }
+    additionalSerializersInScopeOfCurrentFile[kType.toClassDescriptor]?.let { return it }
     if (kType in contextualKClassListInCurrentFile) return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     return analyzeSpecialSerializers(module, annotations) ?: findTypeSerializer(module, kType)
 }