Index: src/economy.cpp
===================================================================
--- src/economy.cpp	(revision 11012)
+++ src/economy.cpp	(working copy)
@@ -1802,12 +1802,17 @@
 	Player *p;
 	CommandCost cost;
 
-	/* Check if buying shares is allowed (protection against modified clients */
-	if (!IsValidPlayer((PlayerID)p1) || !_patches.allow_shares) return CMD_ERROR;
+	/* Check if buying shares is allowed (protection against modified clients) */
+	/* Cannot buy own shares */
+	if (!IsValidPlayer((PlayerID)p1) || !_patches.allow_shares || _current_player == (PlayerID)p1) return CMD_ERROR;
 
-	SET_EXPENSES_TYPE(EXPENSES_OTHER);
 	p = GetPlayer((PlayerID)p1);
 
+	/* Cannot buy shares of non-existent nor bankrupted company */
+	if (!p->is_active) return CMD_ERROR;
+
+	SET_EXPENSES_TYPE(EXPENSES_OTHER);
+
 	/* Protect new companies from hostile takeovers */
 	if (_cur_year - p->inaugurated_year < 6) return_cmd_error(STR_7080_PROTECTED);
 
@@ -1848,12 +1853,17 @@
 	Player *p;
 	Money cost;
 
-	/* Check if buying shares is allowed (protection against modified clients */
-	if (!IsValidPlayer((PlayerID)p1) || !_patches.allow_shares) return CMD_ERROR;
+	/* Check if selling shares is allowed (protection against modified clients) */
+	/* Cannot sell own shares */
+	if (!IsValidPlayer((PlayerID)p1) || !_patches.allow_shares || _current_player == (PlayerID)p1) return CMD_ERROR;
 
-	SET_EXPENSES_TYPE(EXPENSES_OTHER);
 	p = GetPlayer((PlayerID)p1);
 
+	/* Cannot sell shares of non-existent nor bankrupted company */
+	if (!p->is_active) return CMD_ERROR;
+
+	SET_EXPENSES_TYPE(EXPENSES_OTHER);
+
 	/* Those lines are here for network-protection (clients can be slow) */
 	if (GetAmountOwnedBy(p, _current_player) == 0) return CommandCost();