diff --git a/src/emmy/dual.cljc b/src/emmy/dual.cljc
index 16d799dc..2e352910 100644
--- a/src/emmy/dual.cljc
+++ b/src/emmy/dual.cljc
@@ -554,7 +554,9 @@
   ([primal tag]
    (bundle-element primal 1 tag))
   ([primal tangent tag]
-   {:pre [(v/scalar? primal)]}
+   {:pre [(or (v/scalar? primal)
+              ;; TODO
+              (isa? (v/kind primal) ::v/function))]}
    (if (g/zero? tangent)
      primal
      (->Dual tag primal tangent))))
diff --git a/src/emmy/tape.cljc b/src/emmy/tape.cljc
index 3f6e281c..78d9384a 100644
--- a/src/emmy/tape.cljc
+++ b/src/emmy/tape.cljc
@@ -235,10 +235,9 @@
   all leaves recursively replaced by [[TapeCell]] instances."
   [x tag]
   (cond
-    (v/scalar? x)    (make tag x)
+    (or (v/scalar? x) (f/function? x)) (make tag x)
     (s/structure? x) (s/mapr #(tapify % tag) x)
-    (f/function? x)  (u/illegal "Function input not yet supported.")
-    :else            x))
+    :else x))
 
 ;; ### Accessors