diff --git a/src/datomish/transact.cljc b/src/datomish/transact.cljc
index c9f706a8..b2db09d5 100644
--- a/src/datomish/transact.cljc
+++ b/src/datomish/transact.cljc
@@ -115,6 +115,9 @@
         v (if (ds/kw? (db/schema db) a) ;; TODO: decide if this is best.  We could also check for ref and numeric types.
             v
             (db/entid db v))]
+    (when-not (integer? a)
+      (raise "Unknown attribute " a
+             {:form orig :attribute a}))
     [op e a v tx]))
 
 (defrecord Transaction [db tempids entities])