diff --git a/masque/builder/utils.py b/masque/builder/utils.py
index f23e8d2..51d5e8b 100644
--- a/masque/builder/utils.py
+++ b/masque/builder/utils.py
@@ -111,9 +111,9 @@ def ell(
 
     is_horizontal = numpy.isclose(rotations[0] % pi, 0)
     if bound_type in ('ymin', 'ymax') and is_horizontal:
-        raise BuildError('Asked for {bound_type} position but ports are pointing along the x-axis!')
+        raise BuildError(f'Asked for {bound_type} position but ports are pointing along the x-axis!')
     elif bound_type in ('xmin', 'xmax') and not is_horizontal:
-        raise BuildError('Asked for {bound_type} position but ports are pointing along the y-axis!')
+        raise BuildError(f'Asked for {bound_type} position but ports are pointing along the y-axis!')
 
     direction = rotations[0] + pi                        # direction we want to travel in (+pi relative to port)
     rot_matrix = rotation_matrix_2d(-direction)