diff --git a/src/interface_implementations/ballandbeam.jl b/src/interface_implementations/ballandbeam.jl
index 1764d1fe68284fa16ce2d761a2e7e0fc45e9a4fd..9ae3da6834ea9c2abb390e2b20f44cf0c66a3dcb 100644
--- a/src/interface_implementations/ballandbeam.jl
+++ b/src/interface_implementations/ballandbeam.jl
@@ -99,8 +99,8 @@ function control(p::AbstractBeamOrBallAndBeam, u::AbstractArray)
     send(p.control,u[1])
 end
 control(p::AbstractBeamOrBallAndBeam, u::Number) = send(p.control,u)
-control(p::BeamSimulator, u)             = p.s(u)
-control(p::BallAndBeamSimulator, u)      = error("Not yet implemented")
+control(p::BeamSimulator, u::Number)             = p.s(u)
+control(p::BallAndBeamSimulator, u::Number)      = error("Not yet implemented")
 
 measure(p::Beam)                         = read(p.measure)
 measure(p::BallAndBeam)                  = [read(p.measure1), read(p.measure2)]