diff --git a/lib/redmine/wiki_formatting/textile/redcloth3.rb b/lib/redmine/wiki_formatting/textile/redcloth3.rb
index 80e0a3626..c9bdd0aeb 100644
--- a/lib/redmine/wiki_formatting/textile/redcloth3.rb
+++ b/lib/redmine/wiki_formatting/textile/redcloth3.rb
@@ -1020,11 +1020,12 @@ class RedCloth3 < String
     end
 
     def flush_left( text )
-        indt = 0
-        if text =~ /^ /
-            unless text.empty?
-                indt += 1 while text !~ /^ {#{indt}}[^ ]/
-            end
+        if /(?![\r\n\t ])[[:cntrl:]]/.match?(text)
+            text.gsub!(/(?![\r\n\t ])[[:cntrl:]]/, '')
+        end
+        if /^ +\S/.match?(text)
+            indt = 0
+            indt += 1 while !/^ {#{indt}}\S/.match?(text)
             if indt.nonzero?
                 text.gsub!( /^ {#{indt}}/, '' )
             end
diff --git a/test/unit/lib/redmine/wiki_formatting/textile_formatter_test.rb b/test/unit/lib/redmine/wiki_formatting/textile_formatter_test.rb
index 19128524e..2358ded58 100644
--- a/test/unit/lib/redmine/wiki_formatting/textile_formatter_test.rb
+++ b/test/unit/lib/redmine/wiki_formatting/textile_formatter_test.rb
@@ -171,6 +171,24 @@ class Redmine::WikiFormatting::TextileFormatterTest < ActionView::TestCase
       </ol>
     EXPECTED
     assert_equal expected.gsub(%r{\s+}, ''), to_html(raw).gsub(%r{\s+}, '')
+
+    raw = <<~RAW
+      * Item-1
+
+        * Item-1a
+        * Item-1b
+    RAW
+    expected = <<~EXPECTED
+      <ul>
+        <li>Item-1
+          <ul>
+            <li>Item-1a</li>
+            <li>Item-1b</li>
+          </ul>
+        </li>
+      </ul>
+    EXPECTED
+    assert_equal expected.gsub(%r{\s+}, ''), to_html(raw).gsub(%r{\s+}, '')
   end
 
   def test_escaping