diff --git a/gtfs/spec/en/reference.md b/gtfs/spec/en/reference.md
index 04a7904e1..d6650fc83 100644
--- a/gtfs/spec/en/reference.md
+++ b/gtfs/spec/en/reference.md
@@ -310,7 +310,7 @@ File: **Optional**
 |  `trip_id` | ID referencing `trips.trip_id` | **Required** | Identifies a trip to which the specified headway of service applies. |
 |  `start_time` | Time | **Required** | Time at which the first vehicle departs from the first stop of the trip with the specified headway. |
 |  `end_time` | Time | **Required** | Time at which service changes to a different headway (or ceases) at the first stop in the trip. |
-|  `headway_secs` | Non-negative integer | **Required** | Time, in seconds, between departures from the same stop (headway) for the trip, during the time interval specified by `start_time` and `end_time`. Multiple headways for the same trip are allowed, but may not overlap. New headways may start at the exact time the previous headway ends.  |
+|  `headway_secs` | Positive integer | **Required** | Time, in seconds, between departures from the same stop (headway) for the trip, during the time interval specified by `start_time` and `end_time`. Multiple headways for the same trip are allowed, but may not overlap. New headways may start at the exact time the previous headway ends.  |
 |  `exact_times` | Enum | Optional | Indicates the type of service for a trip. See the file description for more information. Valid options are:<br><br>`0` or empty - Frequency-based trips.<br>`1` - Schedule-based trips with the exact same headway throughout the day. In this case the `end_time` value must be greater than the last desired trip `start_time` but less than the last desired trip start_time + `headway_secs`. |
 
 ### transfers.txt