 					case let .Value(value):
 						foundValue = value
 						continue argumentLoop
-					
+
 					default:
 						break
 					}
 				}
 
-				let description = "Missing value for argument --\(key)"
-				return failure(CarthageError.InvalidArgument(description: description).error)
+				return failure(missingArgumentError("--\(key)"))
 			} else {
 				rawArguments.append(arg)
 			}

			return "\"\(value)\""
		}
	}
}

/// Destructively parses a list of command-line arguments.

		} else if let defaultValue = option.defaultValue {
			return success(defaultValue)
		} else {
			return failure(missingArgumentError(option.description))
		}

	case .Usage:
