diff --git a/src/command_func.h b/src/command_func.h
--- a/src/command_func.h
+++ b/src/command_func.h
@@ -34,10 +34,10 @@ static const CommandCost CMD_ERROR = Com
  */
 #define return_cmd_error(errcode) return CommandCost(errcode);
 
-CommandCost DoCommand(TileIndex tile, uint32 p1, uint32 p2, DoCommandFlag flags, uint32 cmd, const char *text = NULL);
+CommandCost DoCommand(TileIndex tile, uint32 p1, uint32 p2, DoCommandFlag flags, uint32 cmd, const char *text = nullptr);
 CommandCost DoCommand(const CommandContainer *container, DoCommandFlag flags);
 
-bool DoCommandP(TileIndex tile, uint32 p1, uint32 p2, uint32 cmd, CommandCallback *callback = NULL, const char *text = NULL, bool my_cmd = true);
+bool DoCommandP(TileIndex tile, uint32 p1, uint32 p2, uint32 cmd, CommandCallback *callback = nullptr, const char *text = nullptr, bool my_cmd = true);
 bool DoCommandP(const CommandContainer *container, bool my_cmd = true);
 
 CommandCost DoCommandPInternal(TileIndex tile, uint32 p1, uint32 p2, uint32 cmd, CommandCallback *callback, const char *text, bool my_cmd, bool estimate_only);