diff --git a/src/build123d/objects_curve.py b/src/build123d/objects_curve.py
index ae1e345e..d4383b3e 100644
--- a/src/build123d/objects_curve.py
+++ b/src/build123d/objects_curve.py
@@ -200,9 +200,9 @@ def __init__(
         # Protect against massive circles that are effectively straight lines
         max_size = 2 * other.bounding_box().add(arc_pt).diagonal
 
-        # Function to be minimized
+        # Function to be minimized - note radius is a numpy array
         def func(radius, perpendicular_bisector):
-            center = arc_pt + perpendicular_bisector * radius
+            center = arc_pt + perpendicular_bisector * radius[0]
             separation = other.distance_to(center)
             return abs(separation - radius)