diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6575875..1cd7688 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ To see every change with descriptions aimed at developers, see
 As a continuously updated web app, Cocreate uses dates
 instead of version numbers.
 
+## 2023-11-13
+
+* Fix zero-length arrows, in particular fixing crash on Firefox
+
 ## 2023-11-03
 
 * Fix arrowheads getting clipped when downloading SVG/PDF,
diff --git a/client/lib/dom.coffee b/client/lib/dom.coffee
index 0bd64a3..125eb43 100644
--- a/client/lib/dom.coffee
+++ b/client/lib/dom.coffee
@@ -108,7 +108,13 @@ export svgArrowCoords = (pEnd, pAdj, width) ->
   else
     dx = pAdj.x - x
     dy = pAdj.y - y
-  scale = width / Math.sqrt dx*dx + dy*dy
+  len = Math.sqrt dx*dx + dy*dy
+  ## Zero-length arrows are oriented rightward in Chrome and Firefox.
+  if len < 0.0001
+    dx = -1
+    dy = 0
+    len = 1
+  scale = width / len
   dx *= scale
   dy *= scale
   px = -dy