diff --git a/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt b/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt
index f41d1d75f78..91a8f603a80 100644
--- a/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt
+++ b/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt
@@ -156,7 +156,7 @@ open class KotlinPlatformImplementationPluginBase(platformName: String) : Kotlin
 
             val commonSources = getKotlinSourceDirectorySetSafe(commonSourceSet)!!
             platformTask.source(commonSources)
-            platformTask.commonSourceSet = commonSources
+            platformTask.commonSourceSet += commonSources
         }
     }