diff --git a/gsk/gskcontour.c b/gsk/gskcontour.c
index 1bd67ba688..d64d35f381 100644
--- a/gsk/gskcontour.c
+++ b/gsk/gskcontour.c
@@ -1467,6 +1467,8 @@ gsk_circle_contour_new (const graphene_point_t *center,
 {
   GskCircleContour *self;
 
+  g_assert (radius > 0);
+
   self = g_new0 (GskCircleContour, 1);
 
   self->contour.klass = &GSK_CIRCLE_CONTOUR_CLASS;
@@ -1700,7 +1702,10 @@ gsk_rect_contour_get_closest_point (const GskContour       *contour,
   if (gsk_rect_contour_closest_point (self, point, threshold, out_dist, &distance))
     {
       result->idx = 1;
-      result->t = distance / self->length;
+      if (self->length == 0)
+        result->t = 0;
+      else
+        result->t = distance / self->length;
 
       return TRUE;
     }
@@ -1961,7 +1966,10 @@ gsk_rect_contour_get_point (const GskContour *contour,
   const GskRectContour *self = (const GskRectContour *) contour;
 
   result->idx = 1;
-  result->t = CLAMP (distance / self->length, 0, 1);
+  if (self->length == 0)
+    result->t = 0;
+  else
+    result->t = CLAMP (distance / self->length, 0, 1);
 }
 
 static float
diff --git a/gsk/gskpath.c b/gsk/gskpath.c
index 67aa2bbd71..ada80b83d1 100644
--- a/gsk/gskpath.c
+++ b/gsk/gskpath.c
@@ -603,6 +603,7 @@ gsk_path_get_closest_point (GskPath                *self,
       if (gsk_contour_get_closest_point (self->contours[i], point, threshold, res, &distance))
         {
           found = TRUE;
+          g_assert (0 <= res->t && res->t <= 1);
           res->contour = i;
           threshold = distance;
         }
diff --git a/gsk/gskpathmeasure.c b/gsk/gskpathmeasure.c
index 2aced2d12f..cb94ae7cbe 100644
--- a/gsk/gskpathmeasure.c
+++ b/gsk/gskpathmeasure.c
@@ -285,6 +285,9 @@ gsk_path_measure_get_point (GskPathMeasure *self,
   contour = gsk_path_get_contour (self->path, i);
 
   gsk_contour_get_point (contour, self->measures[i].contour_data, distance, res);
+
+  g_assert (0 <= res->t && res->t <= 1);
+
   res->contour = i;
 
   return TRUE;