Index: binaries/data/mods/public/globalscripts/utility.js
===================================================================
--- binaries/data/mods/public/globalscripts/utility.js
+++ binaries/data/mods/public/globalscripts/utility.js
@@ -52,6 +52,38 @@
 }
 
 /**
+ * Check if the data is contained in its stringification.
+ */
+function isEqualToStringified(data)
+{
+	return  deepIncludes(data, JSON.parse(JSON.stringify(data)));
+}
+
+/**
+ * Recursively check if the base is contained in the conatiner.
+ */
+function deepIncludes(base, container)
+{
+	if (typeof base == "object")
+	{
+		for (const key in base)
+			if (!deepIncludes(base[key], container[key]))
+				return false;
+		return true;
+	}
+
+	if (typeof base == "array")
+	{
+		for (let i = 0; i < base.length; ++i)
+			if (!deepIncludes(base[i], container[i]))
+				return false;
+		return true;
+	}
+
+	return base === container;
+}
+
+/**
  * 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/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;