diff --git a/docs/julia/heater_3d.jl b/docs/julia/heater_3d.jl
index f257d59a..f20dd708 100644
--- a/docs/julia/heater_3d.jl
+++ b/docs/julia/heater_3d.jl
@@ -19,6 +19,7 @@
 
 # %% tags=["remove-stderr", "hide-input", "hide-output"]
 using CairoMakie
+using Printf
 
 using Gridap
 using GridapGmsh