diff --git a/src/lang/english.txt b/src/lang/english.txt
--- a/src/lang/english.txt
+++ b/src/lang/english.txt
@@ -1065,6 +1065,7 @@ STR_CITY_APPROVAL_PERMISSIVE            
 STR_CITY_APPROVAL_TOLERANT                                      :Tolerant
 STR_CITY_APPROVAL_HOSTILE                                       :Hostile
 
+STR_WARNING_NO_SUITABLE_AI                                      :{WHITE}No suitable AIs available...{}You can download several AIs via the 'Online Content' system
 STR_WARNING_DIFFICULTY_TO_CUSTOM                                :{WHITE}This action changed the difficulty level to custom
 
 # Advanced settings window
diff --git a/src/settings.cpp b/src/settings.cpp
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -58,6 +58,7 @@
 #include "settings_func.h"
 #include "ini_type.h"
 #include "ai/ai_config.hpp"
+#include "ai/ai.hpp"
 #include "newgrf.h"
 #include "ship.h"
 #include "company_base.h"
@@ -851,6 +852,11 @@ static bool DifficultyChange(int32)
 		_settings_game.difficulty.diff_level = 3;
 	}
 
+	if (((_game_mode == GM_MENU) ? _settings_newgame.difficulty : _settings_game.difficulty).max_no_competitors != 0 &&
+			AI::GetInfoList()->size() == 0 && (!_networking || _network_server)) {
+		ShowErrorMessage(STR_WARNING_NO_SUITABLE_AI, INVALID_STRING_ID, 0, 0, true);
+	}
+
 	/* If we are a network-client, update the difficult setting (if it is open).
 	 * Use this instead of just dirtying the window because we need to load in
 	 * the new difficulty settings */
diff --git a/src/settings_gui.cpp b/src/settings_gui.cpp
--- a/src/settings_gui.cpp
+++ b/src/settings_gui.cpp
@@ -31,6 +31,7 @@
 #include "company_func.h"
 #include "viewport_func.h"
 #include "core/geometry_func.hpp"
+#include "ai/ai.hpp"
 #include <map>
 
 #include "table/sprites.h"
@@ -653,6 +654,11 @@ public:
 			SetDifficultyLevel(3, &this->opt_mod_temp.difficulty); // set difficulty level to custom
 			this->LowerWidget(GDW_LVL_CUSTOM);
 			this->InvalidateData();
+
+			if (widget / 3 == 0 && this->opt_mod_temp.difficulty.max_no_competitors != 0 &&
+					AI::GetInfoList()->size() == 0) {
+				ShowErrorMessage(STR_WARNING_NO_SUITABLE_AI, INVALID_STRING_ID, 0, 0, true);
+			}
 			return;
 		}