diff --git a/src/grid/grid_physics_3d.f90 b/src/grid/grid_physics_3d.f90
index 26578fc6..c3ce4948 100644
--- a/src/grid/grid_physics_3d.f90
+++ b/src/grid/grid_physics_3d.f90
@@ -118,6 +118,13 @@ subroutine setup_grid_physics(group, use_mrw, use_pda, compute_specific_energy_s
     allocate(density(geo%n_cells, n_dust))
     allocate(specific_energy(geo%n_cells, n_dust))
     allocate(specific_energy_spectrum(geo%n_cells,n_dust,n_nu_bins))
+    ! initialize: this array is only fully populated later (from the
+    ! photon deposits, or from the minimum specific energy), but with
+    ! specific_energy_type='additional' it is COPIED into
+    ! specific_energy_additional_spectrum before that happens -- copying
+    ! an uninitialized allocation injects heap garbage into the
+    ! frequency-resolved output at every iteration.
+    specific_energy_spectrum = 0._dp
 
     if(n_dust > 0) then