diff --git a/main-settings/src/main/scala/sbt/Append.scala b/main-settings/src/main/scala/sbt/Append.scala
index 4b2c1987f..bb50f52fa 100644
--- a/main-settings/src/main/scala/sbt/Append.scala
+++ b/main-settings/src/main/scala/sbt/Append.scala
@@ -6,6 +6,8 @@ import scala.annotation.implicitNotFound
 import sbt.internal.util.Attributed
 import Def.Initialize
 import reflect.internal.annotations.compileTimeOnly
+import sbt.internal.io.Source
+import sbt.io.{ AllPassFilter, NothingFilter }
 
 object Append {
   @implicitNotFound(
@@ -96,4 +98,11 @@ object Append {
       def appendValue(a: Seq[T], b: Option[T]): Seq[T] = b.fold(a)(a :+ _)
       def appendValues(a: Seq[T], b: Option[T]): Seq[T] = b.fold(a)(a :+ _)
     }
+  implicit def appendSource: Sequence[Seq[Source], Seq[File], File] =
+    new Sequence[Seq[Source], Seq[File], File] {
+      def appendValue(a: Seq[Source], b: File): Seq[Source] =
+        appendValues(a, Seq(b))
+      def appendValues(a: Seq[Source], b: Seq[File]): Seq[Source] =
+        a ++ b.map(new Source(_, AllPassFilter, NothingFilter))
+    }
 }