diff --git a/build.sbt b/build.sbt
index 818210006..9507d71ac 100644
--- a/build.sbt
+++ b/build.sbt
@@ -656,6 +656,7 @@ lazy val mainProj = (project in file("main"))
     sourceManaged in (Compile, generateContrabands) := baseDirectory.value / "src" / "main" / "contraband-scala",
     testOptions in Test += Tests
       .Argument(TestFrameworks.ScalaCheck, "-minSuccessfulTests", "1000"),
+    Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat, // Delete this after 1.3.0-RC2.
     mimaSettings,
     mimaBinaryIssueFilters ++= Vector(
       // New and changed methods on KeyIndex. internal.