diff --git a/core/src/magic.sld b/core/src/magic.sld
index 6592a28..1453903 100644
--- a/core/src/magic.sld
+++ b/core/src/magic.sld
@@ -388,9 +388,34 @@
      (define (do-build conn item)
        (call/cc
         (lambda (cc)
+         (define (build-done)
+          ; Notify our dependencies that we're done.
+          (mutex-lock! pending-count-mutex)
+          (unless (eq? pending-count 'error)
+            (set! pending-count (- pending-count 1)))
+
+          ; Notify the derivation that it is built.
+          (let* ((meta (derivation-meta (pending-item-ca-drv item)))
+                 (post-build (and meta (assoc 'post-build meta))))
+           (when post-build ((cdr post-build) (pending-item-resolved-paths item))))
+          (mutex-unlock! pending-count-mutex)
+          (mutex-lock! pending-mutex)
+          (set-pending-item-awaiting-count! item 'built)
+          (for-each
+           (lambda (depends-on)
+            (set-pending-item-awaiting-count! depends-on (- (pending-item-awaiting-count depends-on) 1))
+            (wake-up depends-on))
+           (pending-item-awaited-by item))
+          (mutex-unlock! pending-mutex))
+
          (define fallback-drv
           (let* ((meta (derivation-meta (pending-item-ca-drv item)))
-                 (fallback (and meta (assoc 'fallback meta))))
+                 (fallback (and meta (assoc 'fallback meta)))
+                 (result-paths (and meta (assoc 'built-paths meta))))
+           (when result-paths
+             (set-pending-item-resolved-paths! item (cdr result-paths))
+             (build-done)
+             (cc #f))
            (and fallback (cdr fallback))))
          ; Rewrite CA drv to IA drv using the known inputs
          (define new-drvs (list))
@@ -472,24 +497,11 @@
                   (cons o ca-store-path))
             outputs)))
 
-         ; Notify our dependencies that we're done.
-         (mutex-lock! pending-count-mutex)
-         (unless (eq? pending-count 'error)
-           (set! pending-count (- pending-count 1)))
+         (set-derivation-meta! (pending-item-ca-drv item)
+            (cons (cons 'built-paths (pending-item-resolved-paths item))
+              (or (derivation-meta (pending-item-ca-drv item)) '())))
+         (build-done))))
 
-         ; Notify the derivation that it is built.
-         (let* ((meta (derivation-meta ca-drv))
-                (post-build (and meta (assoc 'post-build meta))))
-          (when post-build ((cdr post-build) (pending-item-resolved-paths item))))
-         (mutex-unlock! pending-count-mutex)
-         (mutex-lock! pending-mutex)
-         (set-pending-item-awaiting-count! item 'built)
-         (for-each
-          (lambda (depends-on)
-           (set-pending-item-awaiting-count! depends-on (- (pending-item-awaiting-count depends-on) 1))
-           (wake-up depends-on))
-          (pending-item-awaited-by item))
-         (mutex-unlock! pending-mutex))))
      (mutex-lock! pending-mutex)
      (define root-pend (get-item input-drv))
      (mutex-unlock! pending-mutex)