diff --git a/coloraide/spaces/din99o.py b/coloraide/spaces/din99o.py
index 8b09dd478..11354954c 100644
--- a/coloraide/spaces/din99o.py
+++ b/coloraide/spaces/din99o.py
@@ -39,8 +39,8 @@ def lab_to_din99o(lab: Vector) -> Vector:
     """XYZ to DIN99o."""
 
     l, a, b = lab
-    val = 1 + abs(C2 * l)
-    l99o = C1 * math.copysign(1, l) * math.log(val) / KE
+    val = 1 + C2 * l
+    l99o = C1 * math.log(val) / KE if val >= 0 else math.nan
 
     if a == 0 and b == 0:
         a99o = b99o = 0.0
diff --git a/docs/src/markdown/about/changelog.md b/docs/src/markdown/about/changelog.md
index e6a507369..a75100f76 100644
--- a/docs/src/markdown/about/changelog.md
+++ b/docs/src/markdown/about/changelog.md
@@ -8,6 +8,7 @@
 -   **FIX**: Much more accurate ICtCp matrices.
 -   **FIX**: Fix typing of deeply nested arrays in `algebra`.
 -   **FIX**: Fix issue with HCT undefined channel resolver.
+-   **FIX**: Proper handling of negative lightness for DIN99o.
 
 ## 2.13.1