Index: economy.c
===================================================================
--- economy.c	(revision 11012)
+++ economy.c	(working copy)
@@ -1633,11 +1633,16 @@
 	Player *p;
 	int64 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;
 
+	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);
-	p = GetPlayer(p1);
 
 	/* Protect new companies from hostile takeovers */
 	if (_cur_year - p->inaugurated_year < 6) return_cmd_error(STR_7080_PROTECTED);
@@ -1678,11 +1683,16 @@
 	Player *p;
 	int64 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;
 
+	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);
-	p = GetPlayer(p1);
 
 	/* Those lines are here for network-protection (clients can be slow) */
 	if (GetAmountOwnedBy(p, _current_player) == 0) return 0;