diff --git a/gnu/packages/bootloaders.scm b/gnu/packages/bootloaders.scm
index a3f436373d..98afc6a7c1 100644
--- a/gnu/packages/bootloaders.scm
+++ b/gnu/packages/bootloaders.scm
@@ -240,7 +240,7 @@ (define-public dtc
     (build-system gnu-build-system)
     (native-inputs
      `(("bison" ,bison)
-       ("flex" ,flex)))
+       ("flex" ,flex-2.6.1))) ; A bug in flex prevents building with flex-2.6.3.
     (arguments
      `(#:make-flags
        (list "CC=gcc"