diff --git a/src/readouts/ADCs.jl b/src/readouts/ADCs.jl
index afdad3d8f6c5817ea8f0fa2e924058297d7ca9b1..f5c0c2d0a0e8ef340ec7d69453e2a3e8d9d29fb2 100644
--- a/src/readouts/ADCs.jl
+++ b/src/readouts/ADCs.jl
@@ -21,7 +21,7 @@ Adds a readout event with `nsamples` readouts.
 struct ADC <: BuildingBlock
     nsamples :: Integer
     dwell_time :: VariableType
-    time_to_center :: Union{Nothing, VariableType}
+    time_to_center :: VariableType
 end
 
 function ADC(nsamples; dwell_time=nothing, time_to_center=nothing, center_halfway=true, kwargs...)