diff --git a/guix/build/union.scm b/guix/build/union.scm
index 1b09da4..6e2b296 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -103,8 +103,13 @@ single leaf."
       (leaf leaf))))
 
 (define (file=? file1 file2)
-  "Return #t if the contents of FILE1 and FILE2 are identical, #f otherwise."
-  (and (= (stat:size (stat file1)) (stat:size (stat file2)))
+  "Return #t if FILE1 and FILE2 are regular files and their contents are
+identical, #f otherwise."
+  (let ((st1 (stat file1))
+        (st2 (stat file2)))
+    (and (eq? (stat:type st1) 'regular)
+         (eq? (stat:type st2) 'regular)
+         (= (stat:size st1) (stat:size st2))
          (call-with-input-file file1
            (lambda (port1)
              (call-with-input-file file2
@@ -117,7 +122,7 @@ single leaf."
                          (n2 (get-bytevector-n! port2 buf2 0 len)))
                      (and (equal? n1 n2)
                           (or (eof-object? n1)
-                            (loop)))))))))))
+                              (loop))))))))))))
 
 (define* (union-build output directories
                       #:key (log-port (current-error-port)))