diff --git a/src/main/java/com/conveyal/r5/analyst/WebMercatorGridPointSet.java b/src/main/java/com/conveyal/r5/analyst/WebMercatorGridPointSet.java
index 55a01f028..95a1b6ab6 100644
--- a/src/main/java/com/conveyal/r5/analyst/WebMercatorGridPointSet.java
+++ b/src/main/java/com/conveyal/r5/analyst/WebMercatorGridPointSet.java
@@ -131,6 +131,7 @@ public double getLon(int i) {
     public TIntList getPointsInEnvelope (Envelope envelopeFixedDegrees) {
         // Convert fixed-degree envelope to floating, then to world-scale web Mercator pixels at this grid's zoom level.
         // This is not very DRY since we do something very similar in the constructor and elsewhere.
+        // These are the integer pixel numbers containing the envelope, so iteration below must be inclusive of the max.
         int west = lonToPixel(fixedDegreesToFloating(envelopeFixedDegrees.getMinX()), zoom);
         int east = lonToPixel(fixedDegreesToFloating(envelopeFixedDegrees.getMaxX()), zoom);
         int north = latToPixel(fixedDegreesToFloating(envelopeFixedDegrees.getMaxY()), zoom);
diff --git a/src/test/java/com/conveyal/r5/analyst/WebMercatorExtentsTest.java b/src/test/java/com/conveyal/r5/analyst/WebMercatorExtentsTest.java
index 310166529..e0600cb6e 100644
--- a/src/test/java/com/conveyal/r5/analyst/WebMercatorExtentsTest.java
+++ b/src/test/java/com/conveyal/r5/analyst/WebMercatorExtentsTest.java
@@ -26,7 +26,7 @@ class WebMercatorExtentsTest {
      * digits (as when they are stored in a database or serialized over the wire).
      */
     @ParameterizedTest
-    @ValueSource(ints = {9, 10, 11})
+    @ValueSource(ints = {9, 10, 11, 12})
     void wgsMercatorStability (int zoom) {
         Envelope wgsEnvelope = new Envelope();
         wgsEnvelope.expandToInclude(10.22222222, 45.111111);
@@ -49,7 +49,7 @@ void wgsMercatorStability (int zoom) {
      * one cell (rather than zero cells). Also check an envelope with a tiny nonzero envelope away from cell edges.
      */
     @ParameterizedTest
-    @ValueSource(ints = {9, 10, 11})
+    @ValueSource(ints = {9, 10, 11, 12})
     void singleCellExtents (int zoom) {
         Envelope wgsEnvelope = new Envelope();