diff --git a/daemon/fstrim.c b/daemon/fstrim.c
index 1ad3630b0..527acfd48 100644
--- a/daemon/fstrim.c
+++ b/daemon/fstrim.c
@@ -105,7 +105,13 @@ do_fstrim (const char *path,
 
   r = commandv (&out, &err, argv);
   if (r == -1) {
-    reply_with_error ("%s", err);
+    /* If the error is about the kernel operation not being supported
+     * for this filesystem type, then return errno ENOTSUP here.
+     */
+    if (strstr (err, "discard operation is not supported"))
+      reply_with_error_errno (ENOTSUP, "%s", err);
+    else
+      reply_with_error ("%s", err);
     return -1;
   }
 
diff --git a/generator/actions.ml b/generator/actions.ml
index 954b86076..eccef00a4 100644
--- a/generator/actions.ml
+++ b/generator/actions.ml
@@ -10851,6 +10851,10 @@ filesystem, the host filesystem, qemu and the host kernel.
 If this support isn't present it may give an error or even
 appear to run but do nothing.
 
+In the case where the kernel vfs driver does not support
+trimming, this call will fail with errno set to C<ENOTSUP>.
+Currently this happens when trying to trim FAT filesystems.
+
 See also C<guestfs_zero_free_space>.  That is a slightly
 different operation that turns free space in the filesystem
 into zeroes.  It is valid to call C<guestfs_fstrim> either