diff --git a/src/calibre/utils/formatter.py b/src/calibre/utils/formatter.py
index 544463bf27..bfe34c7e7e 100644
--- a/src/calibre/utils/formatter.py
+++ b/src/calibre/utils/formatter.py
@@ -540,8 +540,8 @@ class _Parser:
                              lambda ln, args: IfNode(ln, args[0], (args[1],), (args[2],))),
         'first_non_empty':  (lambda args: len(args) == 1,
                              lambda ln, args: FirstNonEmptyNode(ln, args)),
-        'assign':           (lambda args: len(args) == 2 and args[0].node_type == Node.NODE_RVALUE,
-                             lambda ln, args: AssignNode(ln, args[0].name, args[1])),
+        'assign':           (lambda args: len(args) == 2 and len(args[0]) == 1 and args[0][0].node_type == Node.NODE_RVALUE,
+                             lambda ln, args: AssignNode(ln, args[0][0].name, args[1])),
         'contains':         (lambda args: len(args) == 4,
                              lambda ln, args: ContainsNode(ln, args)),
         'character':        (lambda args: len(args) == 1,