diff --git a/compiler/fir/fir2ir/jvm-backend/src/org/jetbrains/kotlin/fir/backend/jvm/FirJvmMangleComputer.kt b/compiler/fir/fir2ir/jvm-backend/src/org/jetbrains/kotlin/fir/backend/jvm/FirJvmMangleComputer.kt
index b0cc7bd63ad..0f0ec73ce54 100644
--- a/compiler/fir/fir2ir/jvm-backend/src/org/jetbrains/kotlin/fir/backend/jvm/FirJvmMangleComputer.kt
+++ b/compiler/fir/fir2ir/jvm-backend/src/org/jetbrains/kotlin/fir/backend/jvm/FirJvmMangleComputer.kt
@@ -234,6 +234,10 @@ open class FirJvmMangleComputer(
                     tBuilder.appendSignature(suffix)
                 }
             }
+            is ConeDefinitelyNotNullType -> {
+                // E.g. not-null type parameter in Java
+                mangleType(tBuilder, type.original)
+            }
             else -> error("Unexpected type $type")
         }
     }