diff --git a/compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt b/compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt
index 252c4c1b868..d6fe65b626f 100644
--- a/compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt
+++ b/compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt
@@ -607,7 +607,7 @@ class FirElementSerializer private constructor(
         val builder = ProtoBuf.Type.newBuilder()
 
         when (type) {
-            is ConeDefinitelyNotNullType -> return typeProto(type, toSuper, correspondingTypeRef, isDefinitelyNotNullType = true)
+            is ConeDefinitelyNotNullType -> return typeProto(type.original, toSuper, correspondingTypeRef, isDefinitelyNotNullType = true)
             is ConeKotlinErrorType -> {
                 extension.serializeErrorType(type, builder)
                 return builder