diff --git a/library/src/calc_tables.cpp b/library/src/calc_tables.cpp
index b0446fe8..2a14ef8f 100644
--- a/library/src/calc_tables.cpp
+++ b/library/src/calc_tables.cpp
@@ -8,6 +8,7 @@
 */
 
 #include "calc_tables.hpp"
+#include <bit>
 #include <vector>
 
 #include <pbn.hpp>
@@ -254,6 +255,22 @@ int STDCALL CalcAllTablesN(
   if (count * dealsp->no_of_tables > MAXNOOFTABLES * DDS_STRAINS)
     return RETURN_TOO_MANY_TABLES;
 
+  // Each hand must hold exactly 13 cards. A malformed deal otherwise reaches
+  // the multi-threaded solver, which can read out of bounds and crash the
+  // process -- intermittently, depending on the number of tables and heap
+  // layout -- before the per-board card-count check fires. Reject up front so
+  // batch calls fail cleanly with RETURN_CARD_COUNT, matching the contract the
+  // single-board solver already enforces.
+  for (int m = 0; m < dealsp->no_of_tables; m++)
+    for (int h = 0; h < DDS_HANDS; h++)
+    {
+      int cardsInHand = 0;
+      for (int s = 0; s < DDS_SUITS; s++)
+        cardsInHand += std::popcount(dealsp->deals[m].cards[h][s]);
+      if (cardsInHand != 13)
+        return RETURN_CARD_COUNT;
+    }
+
   int ind = 0;
   int lastIndex = 0;
   resp->no_of_boards = 0;
diff --git a/python/tests/test_calc_tables_regression.py b/python/tests/test_calc_tables_regression.py
index 6a8937d4..11f0f117 100644
--- a/python/tests/test_calc_tables_regression.py
+++ b/python/tests/test_calc_tables_regression.py
@@ -20,5 +20,25 @@ def test_nt_row_index_regression(self) -> None:
         self.assertEqual(len(all_rows), 5)
         self.assertEqual(len(nt_only_rows), 5)
         self.assertEqual(nt_only_rows[4], all_rows[4])
+
+    def test_malformed_deal_in_batch_raises_not_crashes(self) -> None:
+        """A malformed deal (a hand without 13 cards) must raise cleanly, even
+        inside a multi-deal batch. Regression: such a deal previously reached
+        the multi-threaded solver and segfaulted the process (intermittently,
+        depending on batch size) instead of returning RETURN_CARD_COUNT (-14).
+        """
+        valid = "N:976.2.QJ74.JT985 83.JT53.K.AQ7643 QJ4.AQ984.T65.K2 AKT52.K76.A9832.-"
+        # North holds only 12 cards (empty clubs): A5.QJ953.KQ962.(void)
+        malformed = "N:A5.QJ953.KQ962. KJT8.K74..AJ62 632.A6.T74.KQ953 Q974.T82.AJ5.T74"
+
+        # Solo: already rejected by the single-board card-count check.
+        with self.assertRaises(RuntimeError):
+            calc_all_tables_pbn([malformed])
+
+        # In a batch: must also raise, not crash the interpreter.
+        with self.assertRaises(RuntimeError):
+            calc_all_tables_pbn([valid] * 20 + [malformed])
+
+
 if __name__ == "__main__":
     unittest.main()