diff --git a/mhcflurry/flanking_encoding.py b/mhcflurry/flanking_encoding.py
index df69c2f2a8870989bd6e0570ff79f61ffe86a7b3..05efe478ef42475660bd1c88ea19afa7abc9bf67 100644
--- a/mhcflurry/flanking_encoding.py
+++ b/mhcflurry/flanking_encoding.py
@@ -109,7 +109,8 @@ class FlankingEncoding(object):
                 "total peptides with this length." % (
                     error_df.iloc[0].peptide,
                     len(error_df.iloc[0].peptide),
-                    len(error_df)))
+                    len(error_df)),
+                supported_peptide_lengths=(1, peptide_max_length + 1))
 
         if n_flank_length > 0:
             n_flanks = df.n_flank.str.pad(