diff --git a/src/delayedarray/RegularTicks.py b/src/delayedarray/RegularTicks.py
index b622ae2..b037373 100644
--- a/src/delayedarray/RegularTicks.py
+++ b/src/delayedarray/RegularTicks.py
@@ -43,8 +43,10 @@ def __getitem__(self, i: int) -> int:
         Returns:
             Position of tick ``i``.
         """
-        if i >= self._len:
-            raise IndexError("'i' is out of range")
-        elif i < 0:
+        if i < 0:
             i += self._len
+            if i < 0:
+                raise IndexError("'i' is out of range")
+        elif i >= self._len:
+            raise IndexError("'i' is out of range")
         return min(self._final, self._spacing * (i + 1))
diff --git a/tests/test_RegularTicks.py b/tests/test_RegularTicks.py
index 32ea3de..9c73560 100644
--- a/tests/test_RegularTicks.py
+++ b/tests/test_RegularTicks.py
@@ -1,4 +1,5 @@
 import delayedarray
+import pytest
 
 
 def test_RegularTicks():
@@ -8,6 +9,11 @@ def test_RegularTicks():
     assert list(out) == ref
     assert out[-1] == 50 # check reverse indexing.
 
+    with pytest.raises(IndexError, match="out of range") as ex:
+        out[-10]
+    with pytest.raises(IndexError, match="out of range") as ex:
+        out[10]
+
     out = delayedarray.RegularTicks(10, 50)
     ref = list(range(10, 51, 10))
     assert list(out) == ref