diff --git a/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/ir/KotlinJsIrCompilation.kt b/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/ir/KotlinJsIrCompilation.kt
index 20eb5243a1a..e05e1d08769 100644
--- a/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/ir/KotlinJsIrCompilation.kt
+++ b/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/ir/KotlinJsIrCompilation.kt
@@ -31,8 +31,9 @@ class KotlinJsIrCompilation(
 
     override val defaultSourceSetName: String
         get() {
+            val target = target as KotlinJsIrTarget
             return lowerCamelCaseName(
-                if ((target as KotlinJsIrTarget).mixedMode)
+                if (target.mixedMode)
                     target.disambiguationClassifierInPlatform
                 else
                     target.disambiguationClassifier,