diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1598defbd..61493f65d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The public API of this library consists of the functions declared in file
 [h3api.h.in](./src/h3lib/include/h3api.h.in).
 
 ## [Unreleased]
+### Fixed
+- Finding invalid edge boundaries should not crash. (#399)
 
 ## [3.7.0] - 2020-09-28
 ### Added
diff --git a/src/apps/testapps/testH3UniEdge.c b/src/apps/testapps/testH3UniEdge.c
index 5af554f8e..68b8c0545 100644
--- a/src/apps/testapps/testH3UniEdge.c
+++ b/src/apps/testapps/testH3UniEdge.c
@@ -342,4 +342,14 @@ SUITE(h3UniEdge) {
                      "Only one edge was deleted for the pentagon");
         }
     }
+
+    TEST(exactEdgeLength_invalid) {
+        // Test that invalid inputs do not cause crashes.
+        t_assert(H3_EXPORT(exactEdgeLengthRads)(0) == 0,
+                 "Invalid edge has zero length");
+        GeoCoord zero = {0, 0};
+        H3Index h3 = H3_EXPORT(geoToH3)(&zero, 0);
+        t_assert(H3_EXPORT(exactEdgeLengthRads)(h3) == 0,
+                 "Non-edge (cell) has zero edge length");
+    }
 }
diff --git a/src/h3lib/lib/h3UniEdge.c b/src/h3lib/lib/h3UniEdge.c
index 9ae5b9e98..32e7e7e17 100644
--- a/src/h3lib/lib/h3UniEdge.c
+++ b/src/h3lib/lib/h3UniEdge.c
@@ -239,6 +239,12 @@ void H3_EXPORT(getH3UnidirectionalEdgeBoundary)(H3Index edge, GeoBoundary* gb) {
 
     // Get the start vertex for the edge
     int startVertex = vertexNumForDirection(origin, direction);
+    if (startVertex == INVALID_VERTEX_NUM) {
+        // This is not actually an edge (i.e. no valid direction),
+        // so return no vertices.
+        gb->numVerts = 0;
+        return;
+    }
 
     // Get the geo boundary for the appropriate vertexes of the origin. Note
     // that while there are always 2 topological vertexes per edge, the