diff --git a/core/src/magic.sld b/core/src/magic.sld
index a4092d0..f25f8a3 100644
--- a/core/src/magic.sld
+++ b/core/src/magic.sld
@@ -339,6 +339,7 @@
 
      (define pending-mutex (make-mutex))
      (define pending-count 0)
+     (define build-error #f)
      (define build-mutex (make-mutex))
      (define build-condvar (make-condition-variable))
      (define to-build '())
@@ -450,6 +451,7 @@
            (lambda (e)
             (mutex-lock! pending-mutex)
             (set! pending-count 'error)
+            (set! build-error e)
             (mutex-unlock! pending-mutex)
             (condition-variable-broadcast! build-condvar)
             (when (error-object? e)
@@ -487,7 +489,9 @@
 
      (for-each (lambda (t) (thread-join! t)) builder-threads)
      (when (eq? pending-count 'error)
-       (error "CA build failed"))
+       (if build-error
+         (raise build-error)
+         (error "CA build failed")))
      root-pend)
    (define (drv-resolve-ca drv outputs)
      (if (drv-is-ca drv)