diff --git a/gnu/packages/dlang.scm b/gnu/packages/dlang.scm
index a00138fc56..3fc10a7f93 100644
--- a/gnu/packages/dlang.scm
+++ b/gnu/packages/dlang.scm
@@ -687,6 +687,7 @@ needed.")
                  (lambda* (#:key tests? #:allow-other-keys)
                    (when tests?
                      (invoke "bin/rdmd" "rdmd_test.d" "bin/rdmd"
+                             "-m" (if #$(target-64bit?) "64" "32")
                              "--rdmd-default-compiler" "ldmd2"))))
                (replace 'install
                  (lambda* (#:key outputs #:allow-other-keys)