diff --git a/table.lisp b/table.lisp
index a6638e9..2dc9e90 100644
--- a/table.lisp
+++ b/table.lisp
@@ -38,6 +38,10 @@
                      collect (cons (cons var nil) entry))
         finally (return (mapcar 'reverse perms))))
 
+(defconstant +maximum-variables+ 12
+  "The maximum number of variables allowed when including variables in the table
+output.")
+
 (defun create-truth-table (prop &key (vars (discover-variables prop))
                                   (include-intermediate t) (include-vars t))
   "Evaluate PROP with all possible combinations of truth values for its
@@ -49,6 +53,10 @@ if it is excluded, `discover-variables' will be used to generate it."
     ((null vars)
      (list (list (cons prop (eval-proposition prop '())))))
     (t
+     (when (> (length vars) +maximum-variables+)
+       (error 'proposition-eval-error
+              :message (format nil "Got ~A variables when the max is ~A"
+                               (length vars) +maximum-variables+)))
      (loop for perm in (permute-variables vars)
            for (value sub-map) = (multiple-value-list
                                   (eval-proposition prop perm))