diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt
index 3a3532b5cf2..a7c8f215763 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt
@@ -124,7 +124,6 @@ private fun JsIrBackendContext.lower(moduleFragment: IrModuleFragment, dependenc
     MultipleCatchesLowering(this).lower(moduleFragment)
     BridgesConstruction(this).runOnFilesPostfix(moduleFragment)
     TypeOperatorLowering(this).lower(moduleFragment)
-    BlockDecomposerLowering(this).runOnFilesPostfix(moduleFragment)
 
     SecondaryCtorLowering(this).apply {
         constructorProcessorLowering.runOnFilesPostfix(moduleFragment.files + dependencies.flatMap { it.files })
@@ -136,6 +135,7 @@ private fun JsIrBackendContext.lower(moduleFragment: IrModuleFragment, dependenc
         inlineClassUsageLowering.lower(moduleFragment)
     }
     AutoboxingTransformer(this).lower(moduleFragment)
+    BlockDecomposerLowering(this).runOnFilesPostfix(moduleFragment)
 
     ClassReferenceLowering(this).lower(moduleFragment)
     PrimitiveCompanionLowering(this).lower(moduleFragment)
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/ir/IrBuilder.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/ir/IrBuilder.kt
index 093bea91ac2..92d2eeedf55 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/ir/IrBuilder.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/ir/IrBuilder.kt
@@ -177,7 +177,7 @@ object JsIrBuilder {
 
     fun buildVar(
         type: IrType,
-        parent: IrDeclarationParent,
+        parent: IrDeclarationParent?,
         name: String = "tmp",
         isVar: Boolean = false,
         isConst: Boolean = false,
@@ -198,7 +198,7 @@ object JsIrBuilder {
         ).also {
             descriptor.bind(it)
             it.initializer = initializer
-            it.parent = parent
+            if (parent != null) it.parent = parent
         }
     }
 
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/AutoboxingTransformer.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/AutoboxingTransformer.kt
index ca3e4634623..22030fb66c6 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/AutoboxingTransformer.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/AutoboxingTransformer.kt
@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.ir.types.isNothing
 import org.jetbrains.kotlin.ir.types.makeNotNull
 import org.jetbrains.kotlin.ir.util.getInlinedClass
 import org.jetbrains.kotlin.ir.util.isInlined
+import org.jetbrains.kotlin.ir.util.isNullable
 
 
 // Copied and adapted from Kotlin/Native
@@ -64,7 +65,7 @@ class AutoboxingTransformer(val context: JsIrBackendContext) : AbstractValueUsag
             else -> this.type
         }
 
-        // TODO: Default parameters are passed as nulls and they need not to be unboxed. Fix this
+        // // TODO: Default parameters are passed as nulls and they need not to be unboxed. Fix this
         if (actualType.makeNotNull().isNothing())
             return this
 
@@ -80,12 +81,43 @@ class AutoboxingTransformer(val context: JsIrBackendContext) : AbstractValueUsag
             else -> return this
         }
 
-        return JsIrBuilder.buildCall(
-            function,
-            expectedType,
-            typeArguments = listOf(actualType, expectedType)
-        ).also {
-            it.putValueArgument(0, this)
+        return buildSafeCall(this, actualType, expectedType) { arg ->
+            JsIrBuilder.buildCall(
+                function,
+                expectedType,
+                typeArguments = listOf(actualType, expectedType)
+            ).also {
+                it.putValueArgument(0, arg)
+            }
+        }
+    }
+
+    private fun buildSafeCall(
+        arg: IrExpression,
+        actualType: IrType,
+        resultType: IrType,
+        call: (IrExpression) -> IrExpression
+    ): IrExpression {
+        if (!actualType.isNullable())
+            return call(arg)
+        return JsIrBuilder.run {
+            val tmp = buildVar(actualType, parent = null, initializer = arg)
+            val nullCheck = buildIfElse(
+                type = resultType,
+                cond = buildCall(irBuiltIns.eqeqSymbol).apply {
+                    putValueArgument(0, buildGetValue(tmp.symbol))
+                    putValueArgument(1, buildNull(irBuiltIns.nothingNType))
+                },
+                thenBranch = buildNull(irBuiltIns.nothingNType),
+                elseBranch = call(buildGetValue(tmp.symbol))
+            )
+            buildBlock(
+                type = resultType,
+                statements = listOf(
+                    tmp,
+                    nullCheck
+                )
+            )
         }
     }
 
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/InlineClasses.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/InlineClasses.kt
index 1d199a51fd3..92c92b04924 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/InlineClasses.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/InlineClasses.kt
@@ -23,8 +23,20 @@ import org.jetbrains.kotlin.ir.types.isMarkedNullable
 fun IrType.getInlinedClass(): IrClass? {
     if (this is IrSimpleType) {
         val erased = erase(this) ?: return null
-        if (!this.isMarkedNullable() && erased.isInline) {
-            // TODO: Don't box nullable type inline classes with non-nullable underlying type (JS IR BE)
+        if (erased.isInline) {
+            if (this.isMarkedNullable()) {
+                var fieldType: IrType
+                var fieldInlinedClass = erased
+                while (true) {
+                    fieldType = getInlineClassBackingField(fieldInlinedClass).type
+                    if (fieldType.isMarkedNullable()) {
+                        return null
+                    }
+
+                    fieldInlinedClass = fieldType.getInlinedClass() ?: break
+                }
+            }
+
             return erased
         }
     }