diff --git a/src/strings_internal.h b/src/strings_internal.h
index 38f4cad5f0..976dd7524b 100644
--- a/src/strings_internal.h
+++ b/src/strings_internal.h
@@ -83,6 +83,7 @@ public:
 	uint64_t GetNextParameter()
 	{
 		struct visitor {
+			uint64_t operator()(const std::monostate &) { throw std::out_of_range("Attempt to read uninitialised parameter as integer"); }
 			uint64_t operator()(const uint64_t &arg) { return arg; }
 			uint64_t operator()(const std::string &) { throw std::out_of_range("Attempt to read string parameter as integer"); }
 		};
@@ -113,11 +114,12 @@ public:
 	const char *GetNextParameterString()
 	{
 		struct visitor {
+			const char *operator()(const std::monostate &) { throw std::out_of_range("Attempt to read uninitialised parameter as string"); }
 			const char *operator()(const uint64_t &) { throw std::out_of_range("Attempt to read integer parameter as string"); }
 			const char *operator()(const std::string &arg) { return arg.c_str(); }
 		};
 
-		const auto &param = GetNextParameterReference();
+		const auto &param = this->GetNextParameterReference();
 		return std::visit(visitor{}, param.data);
 	}
 
diff --git a/src/strings_type.h b/src/strings_type.h
index 6446e4d653..d749ac7960 100644
--- a/src/strings_type.h
+++ b/src/strings_type.h
@@ -72,7 +72,7 @@ static constexpr StringID SPECSTR_SILLY_NAME = 0x70E5; ///< Special string for s
 static constexpr StringID SPECSTR_ANDCO_NAME = 0x70E6; ///< Special string for Surname & Co company names.
 static constexpr StringID SPECSTR_PRESIDENT_NAME = 0x70E7; ///< Special string for the president's name.
 
-using StringParameterData = std::variant<uint64_t, std::string>;
+using StringParameterData = std::variant<std::monostate, uint64_t, std::string>;
 
 /** The data required to format and validate a single parameter of a string. */
 struct StringParameter {
@@ -82,6 +82,7 @@ struct StringParameter {
 	StringParameter() = default;
 	inline StringParameter(StringParameterData &&data) : data(std::move(data)), type(0) {}
 
+	inline StringParameter(const std::monostate &data) : data(data), type(0) {}
 	inline StringParameter(uint64_t data) : data(data), type(0) {}
 
 	inline StringParameter(const char *data) : data(std::string{data}), type(0) {}