diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
index 2d656476287..efb18eb4662 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
@@ -220,7 +220,7 @@ class Fir2IrConverter(
         irClass: IrClass = classifierStorage.getCachedIrClass(regularClass)!!
     ): IrClass {
         val allDeclarations = mutableListOf<FirDeclaration>().apply {
-            addAll(regularClass.declarations.toMutableList())
+            addAll(regularClass.declarations)
             if (generatorExtensions.isNotEmpty()) {
                 addAll(regularClass.generatedMembers(session))
                 addAll(regularClass.generatedNestedClassifiers(session))
@@ -500,4 +500,4 @@ class Fir2IrConverter(
             return Fir2IrResult(irModuleFragment, components, moduleDescriptor)
         }
     }
-}
\ No newline at end of file
+}