diff --git a/lib/irb/ruby-lex.rb b/lib/irb/ruby-lex.rb
index 5967cdb1ba..02bc548809 100644
--- a/lib/irb/ruby-lex.rb
+++ b/lib/irb/ruby-lex.rb
@@ -34,8 +34,9 @@ class RubyLex
     begin
       result = yield code
     rescue ArgumentError => e
-      if e.message.match?(/unknown encoding name/) && code.match?(/\A(?<shebang>#.*\n)?#\s*coding\s*:.*(?<nl>\n)?/)
-        code = code.gsub(/\A(?<shebang>#.*\n)?#\s*coding\s*:.*(?<nl>\n)?/, "\\k<shebang>#\\k<nl>")
+      magic_comment_regexp = /\A(?<shebang>#.*\n)?#\s*(?:encoding|coding)\s*:.*(?<nl>\n)?/
+      if e.message.match?(/unknown encoding name/) && code.match?(magic_comment_regexp)
+        code = code.gsub(magic_comment_regexp, "\\k<shebang>#\\k<nl>")
         retry
       end
     end
diff --git a/test/irb/test_ruby_lex.rb b/test/irb/test_ruby_lex.rb
index 367d1e204a..bc5b4feaf7 100644
--- a/test/irb/test_ruby_lex.rb
+++ b/test/irb/test_ruby_lex.rb
@@ -139,5 +139,18 @@ module TestIRB
         assert_indenting(lines, row.new_line_spaces, true)
       end
     end
+
+    def test_incomplete_encoding_magic_comment
+      input_with_correct_indents = [
+        Row.new(%q(#encoding:u), nil, 0),
+      ]
+
+      lines = []
+      input_with_correct_indents.each do |row|
+        lines << row.content
+        assert_indenting(lines, row.current_line_spaces, false)
+        assert_indenting(lines, row.new_line_spaces, true)
+      end
+    end
   end
 end