diff --git a/guix/build/syscalls.scm b/guix/build/syscalls.scm
index ed833c10b2..5ce0abbb48 100644
--- a/guix/build/syscalls.scm
+++ b/guix/build/syscalls.scm
@@ -915,10 +915,12 @@ always a positive integer."
 
   (catch 'system-error
     (lambda ()
-      (match (window-size-columns (terminal-window-size port))
-        ;; Things like Emacs shell-mode return 0, which is unreasonable.
-        (0 (fall-back))
-        ((? number? columns) columns)))
+      (if (file-port? port)
+          (match (window-size-columns (terminal-window-size port))
+            ;; Things like Emacs shell-mode return 0, which is unreasonable.
+            (0 (fall-back))
+            ((? number? columns) columns))
+          (fall-back)))
     (lambda args
       (let ((errno (system-error-errno args)))
         (if (= errno ENOTTY)
diff --git a/tests/syscalls.scm b/tests/syscalls.scm
index 1b443be0c8..24ea8f5e60 100644
--- a/tests/syscalls.scm
+++ b/tests/syscalls.scm
@@ -257,4 +257,8 @@
 (test-assert "terminal-columns"
   (> (terminal-columns) 0))
 
+(test-assert "terminal-columns non-file port"
+  (> (terminal-columns (open-input-string "Join us now, share the software!"))
+     0))
+
 (test-end)