diff --git a/compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmStringConcatenationLowering.kt b/compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmStringConcatenationLowering.kt
index feae62c6a2d..f33938df338 100644
--- a/compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmStringConcatenationLowering.kt
+++ b/compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmStringConcatenationLowering.kt
@@ -80,7 +80,7 @@ private fun JvmIrBuilder.normalizeArgument(expression: IrExpression): IrExpressi
             //      upcast to 'String?'
             return irImplicitCast(expression, context.irBuiltIns.stringType.makeNullable())
         }
-        if (!(type as IrSimpleType).hasQuestionMark) {
+        if (type is IrSimpleType && !type.hasQuestionMark || type is IrDefinitelyNotNullType) {
             if (upperBound.isByte() || upperBound.isShort()) {
                 //  Expression type is not null,
                 //  T <: Byte || T <: Short =>
diff --git a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
index 1f78a794d4b..af6f5f856d1 100644
--- a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
+++ b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedArrayType.kt
@@ -1,8 +1,6 @@
 // !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 3a7c2fab450..3eb4952fb33 100644
--- a/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt
+++ b/compiler/testData/codegen/box/unsignedTypes/evaluateConstructorOfUnsignedType.kt
@@ -1,8 +1,6 @@
 // !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