diff --git a/src/strings.cpp b/src/strings.cpp
index eabeb09990..5a3f9295b3 100644
--- a/src/strings.cpp
+++ b/src/strings.cpp
@@ -851,7 +851,7 @@ static std::vector<const char *> _game_script_raw_strings;
  */
 static void FormatString(StringBuilder &builder, const char *str_arg, StringParameters &args, uint case_index, bool game_script, bool dry_run)
 {
-	size_t orig_offset = args.offset;
+	size_t orig_offset = args.GetOffset();
 
 	if (!dry_run && args.HasTypeInformation()) {
 		/*
@@ -876,7 +876,7 @@ static void FormatString(StringBuilder &builder, const char *str_arg, StringPara
 			FormatString(dry_run_builder, str_arg, args, case_index, game_script, true);
 		}
 		/* We have to restore the original offset here to to read the correct values. */
-		args.offset = orig_offset;
+		args.SetOffset(orig_offset);
 	}
 	WChar b = '\0';
 	uint next_substr_case_index = 0;
@@ -1070,7 +1070,7 @@ static void FormatString(StringBuilder &builder, const char *str_arg, StringPara
 			}
 
 			case SCC_ARG_INDEX: { // Move argument pointer
-				args.offset = orig_offset + (byte)*str++;
+				args.SetOffset(orig_offset + (byte)*str++);
 				break;
 			}
 
diff --git a/src/strings_internal.h b/src/strings_internal.h
index 2feb282aa4..208f082a9d 100644
--- a/src/strings_internal.h
+++ b/src/strings_internal.h
@@ -20,9 +20,9 @@ protected:
 	WChar *type;              ///< Array with type information about the data. Can be nullptr when no type information is needed. See #StringControlCode.
 
 	WChar next_type = 0; ///< The type of the next data that is retrieved.
+	size_t offset = 0; ///< Current offset in the data/type arrays.
 
 public:
-	size_t offset = 0; ///< Current offset in the data/type arrays.
 	size_t num_param; ///< Length of the data array.
 
 	/** Create a new StringParameters instance. */
@@ -60,6 +60,31 @@ public:
 	void PrepareForNextRun();
 	void SetTypeOfNextParameter(WChar type) { this->next_type = type; }
 
+	/**
+	 * Get the current offset, so it can be backed up for certain processing
+	 * steps, or be used to offset the argument index within sub strings.
+	 * @return The current offset.
+	 */
+	size_t GetOffset() { return this->offset; }
+
+	/**
+	 * Set the offset within the string from where to return the next result of
+	 * \c GetInt64 or \c GetInt32.
+	 * @param offset The offset.
+	 */
+	void SetOffset(size_t offset)
+	{
+		/*
+		 * The offset must be fewer than the number of parameters when it is
+		 * being set. Unless restoring a backup, then the original value is
+		 * correct as well as long as the offset was not changed. In other
+		 * words, when the offset was already at the end of the parameters and
+		 * the string did not consume any parameters.
+		 */
+		assert(offset < this->num_param || this->offset == offset);
+		this->offset = offset;
+	}
+
 	int64 GetInt64();
 
 	/** Read an int32 from the argument array. @see GetInt64. */