diff --git a/app/src/main/java/com/gen/rxbilling/sample/MainActivity.kt b/app/src/main/java/com/gen/rxbilling/sample/MainActivity.kt
index 167dbb9..373c677 100644
--- a/app/src/main/java/com/gen/rxbilling/sample/MainActivity.kt
+++ b/app/src/main/java/com/gen/rxbilling/sample/MainActivity.kt
@@ -7,6 +7,7 @@ import com.gen.rxbilling.client.RxBilling
 import com.gen.rxbilling.client.RxBillingImpl
 import com.gen.rxbilling.connection.BillingClientFactory
 import com.gen.rxbilling.lifecycle.BillingConnectionManager
+import io.reactivex.android.schedulers.AndroidSchedulers
 import io.reactivex.disposables.CompositeDisposable
 import kotlinx.android.synthetic.main.activity_main.*
 import timber.log.Timber
@@ -72,6 +73,7 @@ class MainActivity : AppCompatActivity() {
     private fun loadPurchases() {
         disposable.add(
                 rxBilling.getPurchases(BillingClient.SkuType.SUBS)
+                        .observeOn(AndroidSchedulers.mainThread())
                         .subscribe({
                             Timber.d("getPurchases $it")
                             tvPurchases.text = it.toString()
@@ -83,6 +85,7 @@ class MainActivity : AppCompatActivity() {
     private fun loadHistory() {
         disposable.add(
                 rxBilling.getPurchaseHistory(BillingClient.SkuType.SUBS)
+                        .observeOn(AndroidSchedulers.mainThread())
                         .subscribe({
                             Timber.d("getPurchaseHistory $it")
                             tvHistory.text = it.toString()
@@ -98,6 +101,7 @@ class MainActivity : AppCompatActivity() {
                                 .setSkusList(listOf("your_id1", "your_id2"))
                                 .setType(BillingClient.SkuType.SUBS)
                                 .build())
+                        .observeOn(AndroidSchedulers.mainThread())
                         .subscribe({
                             Timber.d("loadDetails $it")
                             tvDetails.text = it.toString()
diff --git a/build.gradle b/build.gradle
index 95a2177..b52d508 100644
--- a/build.gradle
+++ b/build.gradle
@@ -3,15 +3,15 @@ apply from: 'dependencies.gradle'
 apply plugin: "com.hellofresh.gradle.deblibs"
 
 buildscript {
-    ext.kotlin_version = '1.4.10'
+    ext.kotlin_version = '1.5.21'
     ext.dokka_version = '0.9.16'
     repositories {
         maven { url 'https://plugins.gradle.org/m2/' }
         google()
-        jcenter()
+        mavenCentral()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:4.0.2'
+        classpath 'com.android.tools.build:gradle:4.2.2'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
         classpath "org.jetbrains.dokka:dokka-android-gradle-plugin:$dokka_version"
         classpath "com.hellofresh.gradle:deblibs:2.2.0"
@@ -24,7 +24,7 @@ buildscript {
 allprojects {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 }
 
diff --git a/dependencies.gradle b/dependencies.gradle
index a0779f3..6c10d9b 100644
--- a/dependencies.gradle
+++ b/dependencies.gradle
@@ -27,7 +27,7 @@ ext {
     rxAndroid2Ver = '2.1.1'
 
     // Billing Client
-    billingClientVer = '3.0.1'
+    billingClientVer = '4.0.0'
 
     // Developer-related
     timberVer = '4.7.1'
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index be585da..e35dc5f 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.7.1-all.zip
diff --git a/rxbilling/build.gradle b/rxbilling/build.gradle
index 6097645..9b71338 100644
--- a/rxbilling/build.gradle
+++ b/rxbilling/build.gradle
@@ -7,7 +7,7 @@ apply plugin: 'org.jetbrains.dokka-android'
 android {
 
     defaultConfig {
-        versionName "3.0.0"
+        versionName "4.0.0"
         minSdkVersion minSdkVer
         targetSdkVersion targetSdkVer
         compileSdkVersion compileSdkVer
diff --git a/rxbilling/src/main/java/com/gen/rxbilling/client/RxBilling.kt b/rxbilling/src/main/java/com/gen/rxbilling/client/RxBilling.kt
index e7dc286..45c5d87 100644
--- a/rxbilling/src/main/java/com/gen/rxbilling/client/RxBilling.kt
+++ b/rxbilling/src/main/java/com/gen/rxbilling/client/RxBilling.kt
@@ -155,11 +155,15 @@ class RxBillingImpl(
     private fun getBoughtItems(@BillingClient.SkuType type: String): Single<List<Purchase>> {
         return connectionFlowable
                 .flatMapSingle {
-                    val purchasesResult = it.queryPurchases(type)
-                    return@flatMapSingle if (isSuccess(purchasesResult.responseCode)) {
-                        Single.just(purchasesResult.purchasesList.orEmpty())
-                    } else {
-                        Single.error(BillingException.fromResult(purchasesResult.billingResult))
+                    Single.create<List<Purchase>> { emitter ->
+                        it.queryPurchasesAsync(type) { billingResult, mutableList ->
+                            if (emitter.isDisposed) return@queryPurchasesAsync
+                            if (isSuccess(billingResult.responseCode)) {
+                                emitter.onSuccess(mutableList)
+                            } else {
+                                emitter.onError(BillingException.fromResult(billingResult))
+                            }
+                        }
                     }
                 }.firstOrError()
     }