diff --git a/fill3/fill3/__init__.py b/fill3/fill3/__init__.py
index 0efe3f2..cd38891 100755
--- a/fill3/fill3/__init__.py
+++ b/fill3/fill3/__init__.py
@@ -185,10 +185,9 @@ class ScrollBar:
         width, height = dimensions
         assert width == 1 or height == 1, (width, height)
         length = width if self._is_horizontal else height
-        interval = [abs(fraction) for fraction in self.interval]
-        assert all(0 <= fraction <= 1 for fraction in interval), interval
+        assert all(0 <= fraction <= 1 for fraction in self.interval), self.interval
         (start_index, start_remainder), (end_index, end_remainder) = \
-            [divmod(fraction * length * 8, 8) for fraction in interval]
+            [divmod(fraction * length * 8, 8) for fraction in self.interval]
         start_index, end_index = int(start_index), int(end_index)
         start_remainder, end_remainder = int(start_remainder), int(end_remainder)
         if start_index == end_index: