diff --git a/src/shadowfinder/shadowfinder.py b/src/shadowfinder/shadowfinder.py
index e78711d..cdfb749 100644
--- a/src/shadowfinder/shadowfinder.py
+++ b/src/shadowfinder/shadowfinder.py
@@ -191,7 +191,7 @@ def find_shadows(self):
 
         pos_obj = get_position(valid_datetimes, valid_lons, valid_lats)
 
-        valid_sun_altitudes = pos_obj["altitude"]  # in radians
+        valid_sun_altitudes = np.asarray(pos_obj["altitude"])  # in radians
 
         # If object height and shadow length are set the sun altitudes are used
         #  to calculate the shadow lengths across the world and then compared to