diff --git a/nuskell/ioutils.py b/nuskell/ioutils.py
index 17c8b70..e129b20 100644
--- a/nuskell/ioutils.py
+++ b/nuskell/ioutils.py
@@ -235,8 +235,8 @@ def pair_table(ss, chars=['.']):
             output_string("constant Fuel * ")
 
         name = cplx.name
-        sequ = cplx.sequence
-        stru = cplx.structure
+        sequ = list(cplx.sequence)
+        stru = list(cplx.structure)
 
         ptab = pair_table(stru)