diff --git a/build.sbt b/build.sbt
index fd363537..26783754 100644
--- a/build.sbt
+++ b/build.sbt
@@ -3,7 +3,9 @@ import xerial.sbt.pack.PackPlugin._
 lazy val psforeverSettings = Seq(
   organization := "net.psforever",
   version := "1.0.2-SNAPSHOT",
-  scalaVersion := "2.13.10",
+  // TODO 2.13.5+ breaks Md5Mac test
+  // possibly due to ListBuffer changes? https://github.com/scala/scala/pull/9257
+  scalaVersion := "2.13.4",
   Global / cancelable := false,
   semanticdbEnabled := true,
   semanticdbVersion := scalafixSemanticdb.revision,