diff --git a/spine/ana/script/save.py b/spine/ana/script/save.py
index 45386350..5b3b44da 100644
--- a/spine/ana/script/save.py
+++ b/spine/ana/script/save.py
@@ -23,7 +23,7 @@ class SaveAna(AnaBase):
             'reco_fragment': RecoFragment(), 'truth_fragment': TruthFragment(),
             'reco_particle': RecoParticle(), 'truth_particle': TruthParticle(),
             'reco_interaction': RecoInteraction(),
-            'truth_particle': TruthInteraction()
+            'truth_interaction': TruthInteraction()
     }
 
     def __init__(self, obj_type, fragment=None, particle=None, interaction=None,
diff --git a/spine/build/interaction.py b/spine/build/interaction.py
index dae48b2b..3f2aa851 100644
--- a/spine/build/interaction.py
+++ b/spine/build/interaction.py
@@ -140,20 +140,21 @@ def _build_truth(self, truth_particles, neutrinos=None):
                 p.interaction_id = i
 
             # Append the neutrino information, if it is provided
-            if neutrinos is not None:
-                nu_ids = [part.nu_id for part in inter_particles]
-                assert len(np.unique(nu_ids)) == 1, (
-                        "Interaction made up of particles with different "
-                        "neutrino IDs. Must be unique.")
-                if nu_ids[0] > -1:
-                    interaction.attach_neutrino(neutrinos[nu_ids[0]])
+            nu_ids = [part.nu_id for part in inter_particles]
+            assert len(np.unique(nu_ids)) == 1, (
+                    "Interaction made up of particles with different "
+                    "neutrino IDs. Must be unique.")
+            interaction.nu_id = nu_ids[0]
+
+            if neutrinos is not None and nu_ids[0] > -1:
+                interaction.attach_neutrino(neutrinos[nu_ids[0]])
 
             else:
                 anc_pos = [part.ancestor_position for part in inter_particles]
                 anc_pos = np.unique(anc_pos, axis=0)
                 assert len(anc_pos) == 1, (
-                    "Particles making up a true interaction have different "
-                    "ancestor positions.")
+                        "Particles making up a true interaction have "
+                        "different ancestor positions.")
                 interaction.vertex = anc_pos.flatten()
 
             # Append
diff --git a/spine/data/out/interaction.py b/spine/data/out/interaction.py
index 4228fd21..af10fafe 100644
--- a/spine/data/out/interaction.py
+++ b/spine/data/out/interaction.py
@@ -60,7 +60,9 @@ class InteractionBase:
     _fixed_length_attrs = {'vertex': 3}
 
     # Variable-length attributes as (key, dtype) pairs
-    _var_length_attrs = {'particle_ids': np.int32}
+    _var_length_attrs = {
+            'particles': object, 'particle_ids': np.int32
+    }
 
     # Attributes specifying coordinates
     _pos_attrs = ['vertex']
diff --git a/spine/data/out/particle.py b/spine/data/out/particle.py
index cd90de2d..037d6a91 100644
--- a/spine/data/out/particle.py
+++ b/spine/data/out/particle.py
@@ -95,7 +95,9 @@ class ParticleBase:
     }
 
     # Variable-length attributes as (key, dtype) pairs
-    _var_length_attrs = {'fragment_ids': np.int32}
+    _var_length_attrs = {
+            'fragments': object, 'fragment_ids': np.int32
+    }
 
     # Attributes specifying coordinates
     _pos_attrs = ['start_point', 'end_point']
diff --git a/spine/io/write/hdf5.py b/spine/io/write/hdf5.py
index be1509c7..0ad08e1c 100644
--- a/spine/io/write/hdf5.py
+++ b/spine/io/write/hdf5.py
@@ -387,6 +387,11 @@ def __call__(self, data, cfg=None):
             Dictionary containing the complete SPINE configuration
         """
         # If this function has never been called, initialiaze the HDF5 file
+        # TODO: make this nicer?
+        if np.isscalar(data['index']):
+            for k in data:
+                data[k] = [data[k]]
+
         if (not self.ready and
             (not self.append or os.path.isfile(self.file_name))):
             self.create(data, cfg)