diff --git a/tmd/io/io.py b/tmd/io/io.py
index b260679..d965f0d 100644
--- a/tmd/io/io.py
+++ b/tmd/io/io.py
@@ -122,12 +122,13 @@ def load_population(neurons, tree_types=None, name=None):
 
     pop = Population.Population(name=name)
 
-    files2load = [i for i in files if (i.endswith(".h5") or i.endswith(".swc"))]
-
-    for i in files2load:
+    for filename in files:
         try:
-            pop.append_neuron(load_neuron(i, tree_types=tree_types))
+            assert filename.endswith(('.h5', '.swc'))
+            pop.append_neuron(load_neuron(filename, tree_types=tree_types))
+        except AssertionError:
+            raise Warning("{} is not a valid h5 or swc file".format(filename))
         except LoadNeuronError:
-            print('File failed to load: {}'.format(i))
+            print('File failed to load: {}'.format(filename))
 
     return pop