diff --git a/lib/kotlin/cross-test-server/build.gradle.kts b/lib/kotlin/cross-test-server/build.gradle.kts
index 7a0c48b..508b29d 100644
--- a/lib/kotlin/cross-test-server/build.gradle.kts
+++ b/lib/kotlin/cross-test-server/build.gradle.kts
@@ -48,6 +48,12 @@
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit")
 }
 
+kotlin {
+    jvmToolchain {
+        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(8))
+    }
+}
+
 tasks {
     application {
         applicationName = "TestServer"