Index: binaries/data/mods/public/globalscripts/utility.js
===================================================================
--- binaries/data/mods/public/globalscripts/utility.js
+++ binaries/data/mods/public/globalscripts/utility.js
@@ -115,6 +115,14 @@
 }
 
 /**
+ * Check if the data is contained in its stringification.
+ */
+function isEqualToStringified(data)
+{
+	return deepCompare(data, JSON.parse(JSON.stringify(data)));
+}
+
+/**
  * Removes prefixing path from a path or filename, leaving just the file's name (with extension)
  *
  * ie. a/b/c/file.ext -> file.ext
Index: binaries/data/mods/public/gui/session/AutoFormation.js
===================================================================
--- binaries/data/mods/public/gui/session/AutoFormation.js
+++ binaries/data/mods/public/gui/session/AutoFormation.js
@@ -56,7 +56,7 @@
 	 */
 	getDefault()
 	{
-		return this.defaultFormation == NULL_FORMATION ? undefined : this.defaultFormation;
+		return this.defaultFormation == NULL_FORMATION ? null : this.defaultFormation;
 	}
 
 	/**
@@ -65,6 +65,6 @@
 	getNull()
 	{
 		let walkOnly = Engine.ConfigDB_GetValue("user", "gui.session.formationwalkonly") === "true";
-		return walkOnly ? NULL_FORMATION : undefined;
+		return walkOnly ? NULL_FORMATION : null;
 	}
 }
Index: binaries/data/mods/public/simulation/ai/common-api/entity.js
===================================================================
--- binaries/data/mods/public/simulation/ai/common-api/entity.js
+++ binaries/data/mods/public/simulation/ai/common-api/entity.js
@@ -928,7 +928,7 @@
 	},
 
 	"tradeRoute": function(target, source) {
-		Engine.PostCommand(PlayerID, { "type": "setup-trade-route", "entities": [this.id()], "target": target.id(), "source": source.id(), "route": undefined, "queued": false, "pushFront": false });
+		Engine.PostCommand(PlayerID, { "type": "setup-trade-route", "entities": [this.id()], "target": target.id(), "source": source.id(), "route": null, "queued": false, "pushFront": false });
 		return this;
 	},
 
@@ -983,7 +983,7 @@
 			"autocontinue": false,
 			"queued": false,
 			"pushFront": false,
-			"metadata": metadata	// can be undefined
+			"metadata": metadata // TODO filter out undefineds here
 		});
 		return this;
 	},
Index: binaries/data/mods/public/simulation/helpers/Commands.js
===================================================================
--- binaries/data/mods/public/simulation/helpers/Commands.js
+++ binaries/data/mods/public/simulation/helpers/Commands.js
@@ -4,6 +4,12 @@
 
 function ProcessCommand(player, cmd)
 {
+	// Commands can only contain stringifiable data. If things cannot be
+	// stringified, this can yield an OOS on replay, since the replay uses
+	// stringified commands.
+	if (!isEqualToStringified(cmd))
+		error("Found order containing non-stringifiable data: " + JSON.stringify(cmd));
+
 	let cmpPlayer = QueryPlayerIDInterface(player);
 	if (!cmpPlayer)
 		return;