diff --git a/util/collection/src/main/scala/sbt/Settings.scala b/util/collection/src/main/scala/sbt/Settings.scala
index 38d2c2f5f..4266af1b6 100644
--- a/util/collection/src/main/scala/sbt/Settings.scala
+++ b/util/collection/src/main/scala/sbt/Settings.scala
@@ -244,7 +244,12 @@ trait Init[Scope] {
     @deprecated("Use the non-deprecated Undefined factory method.", "0.13.1")
     def this(definingKey: ScopedKey[_], referencedKey: ScopedKey[_], derived: Boolean) = this(fakeUndefinedSetting(definingKey, derived), referencedKey)
   }
-  final class RuntimeUndefined(val undefined: Seq[Undefined]) extends RuntimeException("References to undefined settings at runtime.")
+  final class RuntimeUndefined(val undefined: Seq[Undefined]) extends RuntimeException("References to undefined settings at runtime.") {
+    override def getMessage =
+      super.getMessage + undefined.map { u =>
+        "\n" + u.defining + " referenced from " + u.referencedKey
+      }.mkString
+  }
 
   @deprecated("Use the other overload.", "0.13.1")
   def Undefined(definingKey: ScopedKey[_], referencedKey: ScopedKey[_], derived: Boolean): Undefined =