diff --git a/pyat/at/acceptance/touschek.py b/pyat/at/acceptance/touschek.py
index 4d60178de..7211cdf9a 100644
--- a/pyat/at/acceptance/touschek.py
+++ b/pyat/at/acceptance/touschek.py
@@ -54,7 +54,7 @@ def haissinski(x, cst):
         nus = ep.f_s/ring.revolution_frequency
         cst = (-0.5 * numpy.sqrt(numpy.pi) * bunch_curr * zn /
                (vrf * h * cs * (abs(etac) * espread / nus)**3))
-        bl = numpy.float(bl0*fsolve(haissinski, (1,), args=cst))
+        bl = bl0*fsolve(haissinski, numpy.array([1.0]), args=cst)[0]
     return bl, espread