diff --git a/README.md b/README.md
index 5dd7e1b..a5543e1 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ dependencies {
 ```
 #### **`gradle.properties`**
 ```properties
-kotlinCorVersion=0.3.+
+kotlinCorVersion=0.3.1+
 ```
 
 ## Usage
diff --git a/build.gradle.kts b/build.gradle.kts
index dfa3f13..731e611 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -5,7 +5,7 @@ plugins {
 }
 
 group = "com.crowdproj.kotlin.cor"
-version = "0.3.0"
+version = "0.3.1"
 
 repositories {
     mavenCentral()
@@ -24,6 +24,8 @@ kotlin {
     sourceSets {
         val coroutinesVersion: String by project
 
+        all { languageSettings.optIn("kotlin.RequiresOptIn") }
+
         @Suppress("UNUSED_VARIABLE")
         val commonMain by getting {
             dependencies {
diff --git a/gradle.properties b/gradle.properties
index 490392e..0876e33 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1,4 +1,4 @@
 kotlinVersion=1.6.21
 
-coroutinesVersion=1.6.2
+coroutinesVersion=1.6.1-native-mt
 #  -native-mt
diff --git a/src/commonTest/kotlin/CorBaseTest.kt b/src/commonTest/kotlin/CorBaseTest.kt
index dd156b5..9307a7a 100644
--- a/src/commonTest/kotlin/CorBaseTest.kt
+++ b/src/commonTest/kotlin/CorBaseTest.kt
@@ -4,13 +4,12 @@ import com.crowdproj.kotlin.cor.handlers.chain
 import com.crowdproj.kotlin.cor.handlers.parallel
 import com.crowdproj.kotlin.cor.handlers.worker
 import kotlinx.coroutines.ExperimentalCoroutinesApi
-import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class CorBaseTest {
-    @OptIn(ExperimentalCoroutinesApi::class)
     @Test
     fun createCor() = runTest {
         val ctx = TestContext(some = 0)
diff --git a/src/commonTest/kotlin/CorExceptionTest.kt b/src/commonTest/kotlin/CorExceptionTest.kt
index 1e74305..f41e9ba 100644
--- a/src/commonTest/kotlin/CorExceptionTest.kt
+++ b/src/commonTest/kotlin/CorExceptionTest.kt
@@ -2,12 +2,14 @@ package com.crowdproj.kotlin.cor
 
 import com.crowdproj.kotlin.cor.handlers.chain
 import com.crowdproj.kotlin.cor.handlers.worker
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.test.runTest
 import kotlin.js.JsName
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNotEquals
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class CorExceptionTest {
 
     @Test
@@ -63,7 +65,7 @@ class CorExceptionTest {
                         throw e
                     }
                 }
-                except { e -> text = "${e.message}$testText2" ?: "" }
+                except { e -> text = "${e.message}$testText2" }
             }
         }.build()
 
diff --git a/src/commonTest/kotlin/CorTest.kt b/src/commonTest/kotlin/CorTest.kt
index 921e27e..e6620ce 100644
--- a/src/commonTest/kotlin/CorTest.kt
+++ b/src/commonTest/kotlin/CorTest.kt
@@ -1,10 +1,12 @@
 package com.crowdproj.kotlin.cor
 
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNotEquals
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class CorTest {
 
     @Test
diff --git a/src/commonTest/kotlin/LoopCorBaseTest.kt b/src/commonTest/kotlin/LoopCorBaseTest.kt
index 680edcd..463227c 100644
--- a/src/commonTest/kotlin/LoopCorBaseTest.kt
+++ b/src/commonTest/kotlin/LoopCorBaseTest.kt
@@ -3,10 +3,12 @@ package com.crowdproj.kotlin.cor
 import com.crowdproj.kotlin.cor.handlers.loopUntil
 import com.crowdproj.kotlin.cor.handlers.loopWhile
 import com.crowdproj.kotlin.cor.handlers.worker
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class LoopCorBaseTest {
     @Test
     fun loopUntil() = runTest {