diff --git a/compiler/ir/ir.tree.persistent/generator/src/org/jetbrains/kotlin/ir/persistentIrGenerator/PersistentIrGenerator.kt b/compiler/ir/ir.tree.persistent/generator/src/org/jetbrains/kotlin/ir/persistentIrGenerator/PersistentIrGenerator.kt
index b0a51cb67a6..95a0701bf16 100644
--- a/compiler/ir/ir.tree.persistent/generator/src/org/jetbrains/kotlin/ir/persistentIrGenerator/PersistentIrGenerator.kt
+++ b/compiler/ir/ir.tree.persistent/generator/src/org/jetbrains/kotlin/ir/persistentIrGenerator/PersistentIrGenerator.kt
@@ -274,7 +274,7 @@ internal object PersistentIrGenerator {
 
         protoFields += fields.mapNotNull { f ->
             f.proto?.protoPrefix?.let { p ->
-                val modifier = f.proto.fieldKind.toString().toLowerCase()
+                val modifier = f.proto.fieldKind.toString().lowercase()
                 "$modifier $p ${f.name}"
             }
         }
@@ -708,4 +708,4 @@ internal object PersistentIrGenerator {
             )()
         })
     }
-}
\ No newline at end of file
+}