diff --git a/src/containers/building_blocks.jl b/src/containers/building_blocks.jl
index d409092bdd90b81068ae622803804dd11f8e6881..1bad9d725505aac7d36c0cddde33c02928bd5e5c 100644
--- a/src/containers/building_blocks.jl
+++ b/src/containers/building_blocks.jl
@@ -142,7 +142,7 @@ function start_time(building_block::BaseBuildingBlock, index)
     error("Building block with index '$index' not found")
 end
 
-@defvar duration(bb::BaseBuildingBlock) = sum([variables.duration(wv) for (_, wv) in waveform_sequence(bb)])
+@defvar duration(bb::BaseBuildingBlock) = sum([variables.duration(wv) for (_, wv) in waveform_sequence(bb)]; init=0.)
 
 # Pathway support
 """