diff --git a/core/src/test/scala/com/evolutiongaming/catshelper/LogSpec.scala b/core/src/test/scala/com/evolutiongaming/catshelper/LogSpec.scala
index efeb5967..740fd081 100644
--- a/core/src/test/scala/com/evolutiongaming/catshelper/LogSpec.scala
+++ b/core/src/test/scala/com/evolutiongaming/catshelper/LogSpec.scala
@@ -120,6 +120,21 @@ class LogSpec extends AnyFunSuite with Matchers {
     ))
   }
 
+  test("preset MDC concatenates with in-place MDC") {
+
+        val stateT = for {
+      log0 <- logOf("source")
+      log = log0.withMdc(Log.Mdc.Eager("preset" -> "value"))
+      _ <- log.info("info", Log.Mdc.Eager("info" -> "value"))
+    } yield {}
+
+        val (state, _) = stateT.run(State(Nil))
+    state shouldEqual State(List(
+      Action.Info("info", Log.Mdc.Eager("info" -> "value", "preset" -> "value")),
+      Action.OfStr("source")
+    ))
+  }
+
   test("MDC cleanup") {
 
     val io = for {