diff --git a/coloraide/algebra.py b/coloraide/algebra.py
index 9d1c8935..859ace92 100644
--- a/coloraide/algebra.py
+++ b/coloraide/algebra.py
@@ -71,6 +71,14 @@
 ################################
 # General math
 ################################
+def sign(x: float) -> float:
+    """Return the sign of a given value."""
+
+    if x and x == x:
+        return x / abs(x)
+    return x
+
+
 def order(x: float) -> int:
     """Get the order of magnitude of a number."""
 
diff --git a/coloraide/temperature/ohno_2013.py b/coloraide/temperature/ohno_2013.py
index 7b26dac2..91f2700a 100644
--- a/coloraide/temperature/ohno_2013.py
+++ b/coloraide/temperature/ohno_2013.py
@@ -224,7 +224,7 @@ def to_cct(
         x = (dp ** 2 - dn ** 2 + l ** 2) / (2 * l)
         t = tp + (tn - tp) * (x / l)
         vtx = vp + (vn - vp) * (x / l)
-        sign = math.copysign(1, v - vtx)
+        sign = alg.sign(v - vtx)
         duv = (dp ** 2 - x ** 2) ** (1 / 2) * sign
 
         # Parabolic solution