diff --git a/trycycler/cluster.py b/trycycler/cluster.py
index 3f59969..33f9879 100644
--- a/trycycler/cluster.py
+++ b/trycycler/cluster.py
@@ -80,6 +80,8 @@ def check_input_assemblies(filenames):
         seqs = load_fasta(f)
         contig_names = set()
         for contig_name, _ in seqs:
+            if '/' in contig_name:
+                sys.exit(f'\nError: contig names cannot contain a slash character: {contig_name}')
             if contig_name in contig_names:
                 sys.exit(f'\nError: duplicate contig name: {contig_name}')
             contig_names.add(contig_name)
diff --git a/trycycler/version.py b/trycycler/version.py
index cb7791f..38f40e6 100644
--- a/trycycler/version.py
+++ b/trycycler/version.py
@@ -14,4 +14,4 @@
 If not, see <http://www.gnu.org/licenses/>.
 """
 
-__version__ = '0.4.1'
+__version__ = '0.4.3'