diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/irTypes.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/irTypes.kt
index bb20c1ca327..ac0ebd1ce02 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/irTypes.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/irTypes.kt
@@ -81,10 +81,14 @@ fun IrType.removeAnnotations(): IrType =
     }
 
 val IrType.classifierOrFail: IrClassifierSymbol
-    get() = cast<IrSimpleType>().classifier
+    get() = classifierOrNull ?: error("Can't get classifier of ${render()}")
 
 val IrType.classifierOrNull: IrClassifierSymbol?
-    get() = safeAs<IrSimpleType>()?.classifier
+    get() = when (this) {
+        is IrSimpleType -> classifier
+        is IrDefinitelyNotNullType -> original.classifierOrNull
+        else -> null
+    }
 
 val IrType.classOrNull: IrClassSymbol?
     get() =
diff --git a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
index af6f5f856d1..1f78a794d4b 100644
--- a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
+++ b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
@@ -1,6 +1,8 @@
 // !LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
 // WITH_REFLECT
 // TARGET_BACKEND: JVM
+// KT-51458:
+// IGNORE_BACKEND_FIR: JVM_IR
 
 annotation class AnnoUB(val ub: UByteArray)
 annotation class AnnoUS(val us: UShortArray)
diff --git a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt
index 3eb4952fb33..3a7c2fab450 100644
--- a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt
+++ b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt
@@ -1,6 +1,8 @@
 // !LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
 // WITH_REFLECT
 // TARGET_BACKEND: JVM
+// KT-51458:
+// IGNORE_BACKEND_FIR: JVM_IR
 
 // TODO: it's not clear why compilation fails for Android
 // IGNORE_BACKEND: ANDROID