diff --git a/modules/io/src/main/scala/de/nowchess/io/pgn/PgnExporter.scala b/modules/io/src/main/scala/de/nowchess/io/pgn/PgnExporter.scala
index e16c49a..326b43b 100644
--- a/modules/io/src/main/scala/de/nowchess/io/pgn/PgnExporter.scala
+++ b/modules/io/src/main/scala/de/nowchess/io/pgn/PgnExporter.scala
@@ -22,7 +22,15 @@ object PgnExporter extends GameContextExport:
     var ctx = GameContext.initial
     for move <- context.moves do
       val color = ctx.turn
-      val pieceType = ctx.board.pieceAt(move.from).map(_.pieceType).getOrElse(PieceType.Pawn)
+      val pieceType = ctx.board.pieceAt(move.from)
+        .map(_.pieceType)
+        .getOrElse {
+          val fromStr = move.from.toString
+          val boardStr = ctx.board.pieces.keys.map(_.toString).mkString(",")
+          throw new IllegalStateException(
+            s"Invariant violation: no piece at $fromStr during PGN export replay. Board squares: $boardStr"
+          )
+        }
       val isCapture = ctx.board.pieceAt(move.to).isDefined || move.moveType == MoveType.EnPassant
       val castleSide = move.moveType match
         case MoveType.CastleKingside  => Some("Kingside")