diff --git a/particles/ramp.c b/particles/ramp.c
index c8c2fd5..31df037 100644
--- a/particles/ramp.c
+++ b/particles/ramp.c
@@ -33,13 +33,21 @@ instantiate(const struct particle *particle, const struct tag_set *tags)
     const struct tag *tag = tag_for_name(tags, ramp->tag);
     assert(tag != NULL);
 
+    assert(ramp->count > 0);
+
     long value = tag->as_int(tag);
     long min = tag->min(tag);
     long max = tag->max(tag);
 
-    double progress = (double)value / (max - min);
+    assert(value >= min && value <= max);
+    assert(max >= min);
 
-    size_t idx = progress * ramp->count;
+    size_t idx = 0;
+    if (max - min > 0)
+        idx = ramp->count * value / (max - min);
+
+    if (idx == ramp->count)
+        idx--;
     /*
      * printf("ramp: value: %lu, min: %lu, max: %lu, progress: %f, idx: %zu\n",
      *        value, min, max, progress, idx);