diff --git a/src/ussa1976/core.py b/src/ussa1976/core.py
index 85528bc..42f6dc7 100644
--- a/src/ussa1976/core.py
+++ b/src/ussa1976/core.py
@@ -855,7 +855,7 @@ def thermal_diffusion_coefficient(
 
     Returns
     -------
-    quantity
+    array
         Thermal diffusion coefficient [m^2 / s].
     """
     k = (a / background) * np.power(temperature / 273.15, b)