diff --git a/src/optionstr.c b/src/optionstr.c
index 7c589e9..d7cb38a 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -3330,7 +3330,12 @@
 	if (wid && *s == '(' && (errmsg = check_stl_option(p_ruf)) == NULL)
 	    ru_wid = wid;
 	else
-	    errmsg = check_stl_option(p_ruf);
+	{
+	    // Validate the flags in 'rulerformat' only if it doesn't point to
+	    // a custom function ("%!" flag).
+	    if ((*varp)[1] != '!')
+		errmsg = check_stl_option(p_ruf);
+	}
     }
     // check 'statusline' or 'tabline' only if it doesn't start with "%!"
     else if (rulerformat || s[0] != '%' || s[1] != '!')
diff --git a/src/testdir/dumps/Test_rulerformat_function.dump b/src/testdir/dumps/Test_rulerformat_function.dump
new file mode 100644
index 0000000..a93fd32
--- /dev/null
+++ b/src/testdir/dumps/Test_rulerformat_function.dump
@@ -0,0 +1,2 @@
+> +0&#ffffff0@39
+@22|1|0|,|2|0| @8|3|0|%| 
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 9aec012..2fbce74 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -3867,6 +3867,27 @@
   call StopVimInTerminal(buf)
 endfunc
 
+" Test for using "%!" in 'rulerformat' to use a function
+func Test_rulerformat_function()
+  CheckScreendump
+
+  let lines =<< trim END
+    func TestRulerFn()
+      return '10,20%=30%%'
+    endfunc
+  END
+  call writefile(lines, 'Xrulerformat_function', 'D')
+
+  let buf = RunVimInTerminal('-S Xrulerformat_function', #{rows: 2, cols: 40})
+  call term_sendkeys(buf, ":set ruler rulerformat=%!TestRulerFn()\<CR>")
+  call term_sendkeys(buf, ":redraw!\<CR>")
+  call term_wait(buf)
+  call VerifyScreenDump(buf, 'Test_rulerformat_function', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_getcompletion_usercmd()
   command! -nargs=* -complete=command TestCompletion echo <q-args>
 
diff --git a/src/version.c b/src/version.c
index 2416ad1..243e348 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    890,
+/**/
     889,
 /**/
     888,