diff --git a/main/src/main/scala/sbt/internal/Act.scala b/main/src/main/scala/sbt/internal/Act.scala
index f4ad46e2e..f3da90b61 100644
--- a/main/src/main/scala/sbt/internal/Act.scala
+++ b/main/src/main/scala/sbt/internal/Act.scala
@@ -78,11 +78,16 @@ object Act {
       selectFromValid(ss filter isValid(data), default)
     }
   def selectFromValid(ss: Seq[ParsedKey], default: Parser[ParsedKey])(implicit show: Show[ScopedKey[_]]): Parser[ParsedKey] =
-    selectByTask(selectByConfig(ss)) match {
-      case Seq()       => default
-      case Seq(single) => success(single)
-      case multi       => failure("Ambiguous keys: " + showAmbiguous(keys(multi)))
+    selectByTask(selectByConfig(ss)) partition isBuildKey match {
+      case (_, Seq(single))         => success(single)
+      case (Seq(single), Seq())     => success(single)
+      case (Seq(), Seq())           => default
+      case (buildKeys, projectKeys) => failure("Ambiguous keys: " + showAmbiguous(keys(buildKeys ++ projectKeys)))
     }
+  private def isBuildKey(parsed: ParsedKey): Boolean = parsed.key.scope.project match {
+    case Select(_: BuildReference) => true
+    case _                         => false
+  }
   private[this] def keys(ss: Seq[ParsedKey]): Seq[ScopedKey[_]] = ss.map(_.key)
   def selectByConfig(ss: Seq[ParsedKey]): Seq[ParsedKey] =
     ss match {