diff --git a/klaxon/src/main/kotlin/com/beust/klaxon/JsonObject.kt b/klaxon/src/main/kotlin/com/beust/klaxon/JsonObject.kt
index dfd0395..936b240 100644
--- a/klaxon/src/main/kotlin/com/beust/klaxon/JsonObject.kt
+++ b/klaxon/src/main/kotlin/com/beust/klaxon/JsonObject.kt
@@ -31,7 +31,8 @@ by map {
                 indent(result, level + 1)
             }
 
-            result.append(Render.renderString(k)).append(":")
+            // Do not remove k::toString, it allows any data type (that needs to be casted to a string before), without crashing
+            result.append(Render.renderString(k.toString())).append(":")
             if (prettyPrint && !canonical) {
                 result.append(" ")
             }
diff --git a/klaxon/src/test/kotlin/com/beust/klaxon/Issue340Test.kt b/klaxon/src/test/kotlin/com/beust/klaxon/Issue340Test.kt
new file mode 100644
index 0000000..cca22c3
--- /dev/null
+++ b/klaxon/src/test/kotlin/com/beust/klaxon/Issue340Test.kt
@@ -0,0 +1,27 @@
+package com.beust.klaxon
+
+import org.testng.annotations.Test
+import java.util.*
+import kotlin.test.assertEquals
+
+@Test
+class Issue340Test {
+
+    fun issue340() {
+        val data:Map<Any, Any?> = mapOf(
+            12 to "dummy",
+            "test" to "123",
+            "test2" to 123,
+            23.12 to 12,
+            UUID.fromString("418ef170-f770-4172-8a19-2990e65c6fda") to "abc",
+        )
+
+        val json = JsonObject(data as Map<String, Any?>)
+
+        val jsonString = Klaxon().toJsonString(json)
+
+        val expected = """{"12": "dummy", "test": "123", "test2": 123, "23.12": 12, "418ef170-f770-4172-8a19-2990e65c6fda": "abc"}"""
+
+        assertEquals(jsonString, expected)
+    }
+}