diff --git a/rpg/sequence.py b/rpg/sequence.py
index 3371ed0c1f4238c11243a7b87f48e1342c83fab8..f4d2209b1294ea5bbca1b2e072e68c67dd2053e6 100644
--- a/rpg/sequence.py
+++ b/rpg/sequence.py
@@ -188,6 +188,6 @@ def check_sequence(seq):
     validate = seq.strip().upper()
     for i in validate:
         if i not in core.AMINOACIDS:
-            core.handle_errors("amino acid \"%s\" in %s not recognized." %
-                               (i, validate), 0, "Sequence ")
+            raise ValueError("amino acid \"%s\" in %s not "\
+                             "recognized." % (i, validate))
     return validate