diff --git a/kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Annotations.kt b/kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Annotations.kt
index 96e0f6ac977..45d534c8731 100644
--- a/kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Annotations.kt
+++ b/kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Annotations.kt
@@ -14,7 +14,11 @@ package kotlinx.cinterop
 @Suppress("unused") // Is emitted by the Commonizer
 @Target(AnnotationTarget.TYPEALIAS, AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY)
 @Retention(AnnotationRetention.BINARY)
-@RequiresOptIn(level = RequiresOptIn.Level.WARNING)
+@RequiresOptIn(
+        message = "The declaration is using numbers with different bit widths in least two actual platforms. " +
+                "Such types shall not be used in user-defined 'expect fun' signatures",
+        level = RequiresOptIn.Level.ERROR
+)
 public annotation class UnsafeNumber(val actualPlatformTypes: Array<String>)
 
 /**
diff --git a/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/optimisticCommonization/src/commonMain/kotlin/common.kt b/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/optimisticCommonization/src/commonMain/kotlin/common.kt
index d076ef1f78b..784422c33e0 100644
--- a/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/optimisticCommonization/src/commonMain/kotlin/common.kt
+++ b/libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/optimisticCommonization/src/commonMain/kotlin/common.kt
@@ -1,3 +1,4 @@
 fun testCommon() {
+    @OptIn(kotlinx.cinterop.UnsafeNumber::class)
     intPropertyInterop.unsafeProp
 }