Fix nil error on debugger prompt

lib/irb.rb

@@ -612,10 +612,10 @@ def format_prompt(format, ltype, indent, line_no) # :nodoc:
         when "N"
           @context.irb_name
         when "m"
-          main_str = @context.safe_method_call_on_main(:to_s) rescue "!#{$!.class}"
+          main_str = "#{@context.safe_method_call_on_main(:to_s)}" rescue "!#{$!.class}"
           truncate_prompt_main(main_str)
         when "M"
-          main_str = @context.safe_method_call_on_main(:inspect) rescue "!#{$!.class}"
+          main_str = "#{@context.safe_method_call_on_main(:inspect)}" rescue "!#{$!.class}"
           truncate_prompt_main(main_str)
         when "l"
           ltype

test/irb/test_debugger_integration.rb

@@ -51,6 +51,31 @@ def test_debug
       assert_match(/=>   2\| puts "hello"/, output)
     end
 
+    def test_debug_class_to_s_return_nil
+      write_ruby <<~'ruby'
+        class ToSReturnsNil
+          def to_s
+            nil
+          end
+
+          def do_something
+            binding.irb
+          end
+        end
+
+        ToSReturnsNil.new.do_something
+      ruby
+
+      output = run_ruby_file do
+        type "debug"
+        type "next"
+        type "continue"
+      end
+
+      assert_match(/irb\(\):001> debug/, output)
+      assert_match(/irb:rdbg\(\):002> next/, output)
+    end
+
     def test_debug_command_only_runs_once
       write_ruby <<~'ruby'
         binding.irb