diff --git a/lib/src/back_end/solver.dart b/lib/src/back_end/solver.dart
index 3a5d1876..22bbdde9 100644
--- a/lib/src/back_end/solver.dart
+++ b/lib/src/back_end/solver.dart
@@ -42,7 +42,7 @@ class Solver {
 
     void traverse(Piece piece) {
       // We don't need to worry about selecting pieces that have only one state.
-      if (piece.states.isNotEmpty) unsolvedPieces.add(piece);
+      if (piece.states.length > 1) unsolvedPieces.add(piece);
       piece.forEachChild(traverse);
     }