1 files changed, 1 insertions, 1 deletions
diff --git a/series.mac b/series.mac
index 5617a1e..fd143f4 100644
--- a/series.mac
+++ b/series.mac
@@ -96,7 +96,7 @@ __fill_term_table(table, factors, t_min, t_max, len) := block(
     /* Initialize table row with 0s */
     for j : 0 thru len-1 do table[i, j] : 0,
     /**/
-    if is(op(factors[i+1])="+") then (
+    if not atom(factors[i+1]) and op(factors[i+1])="+" then (
       for term in args(factors[i+1]) do block(
         [j, n],
         n : lopow(term, r),