diff --git a/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala b/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
index 29df005fc856..8968f7196f73 100644
--- a/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
+++ b/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
@@ -59,7 +59,7 @@ object SimpleIdentitySet {
   private class Set3[+Elem <: AnyRef](x0: AnyRef, x1: AnyRef, x2: AnyRef) extends SimpleIdentitySet[Elem] {
     def size = 3
     def + [E >: Elem <: AnyRef](x: E): SimpleIdentitySet[E] =
-      if (contains(this)) this
+      if (contains(x)) this
       else {
         val xs = new Array[AnyRef](4)
         xs(0) = x0
@@ -113,4 +113,4 @@ object SimpleIdentitySet {
       while (i < size) { f(xs(i).asInstanceOf[Elem]); i += 1 }
     }
   }
-}
\ No newline at end of file
+}