diff --git a/build.gradle.kts b/build.gradle.kts
index fd332b2..5d96a9c 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -44,6 +44,15 @@ kotlin {
     jvmToolchain(8)
 }
 
+// backward compat for users on older versions of Kotlin
+// we use "data objects" in test code, so limit this to production code
+tasks.named<org.jetbrains.kotlin.gradle.tasks.KotlinJvmCompile>("compileKotlin") {
+    compilerOptions.apply {
+        apiVersion.set(org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_1_6)
+        languageVersion.set(org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_1_6)
+    }
+}
+
 allprojects {
 
     val varintName = project.name
@@ -119,4 +128,4 @@ allprojects {
             sign(publishing.publications["mavenJava"])
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/main/kotlin/io/exoquery/fansi/Fansi.kt b/src/main/kotlin/io/exoquery/fansi/Fansi.kt
index 7459c72..3decd4c 100644
--- a/src/main/kotlin/io/exoquery/fansi/Fansi.kt
+++ b/src/main/kotlin/io/exoquery/fansi/Fansi.kt
@@ -755,8 +755,8 @@ final class Trie<T>(val strings: List<Pair<String, T>>){
     when {
       list.isEmpty() -> {
         val allChildChars = continuations.map{ (a, _) -> a[0] }
-        val minVal = allChildChars.min()
-        val maxVal = allChildChars.max()
+        val minVal = allChildChars.minOrNull() ?: error("No chars")
+        val maxVal = allChildChars.maxOrNull() ?: error("No chars")
 
         val arrVal = Array<Trie<T>?>(maxVal - minVal + 1, { null })
         for( (char, ss) in continuations.groupBy { (a, _) -> a[0] } ){