diff --git a/prism/extension.c b/prism/extension.c
index d19a004beb..27e799a8da 100644
--- a/prism/extension.c
+++ b/prism/extension.c
@@ -451,6 +451,9 @@ parser_errors(pm_parser_t *parser, rb_encoding *encoding, VALUE source) {
             case PM_ERROR_LEVEL_ARGUMENT:
                 level = ID2SYM(rb_intern("argument"));
                 break;
+            case PM_ERROR_LEVEL_LOAD:
+                level = ID2SYM(rb_intern("load"));
+                break;
             default:
                 rb_raise(rb_eRuntimeError, "Unknown level: %" PRIu8, error->level);
         }
diff --git a/prism/templates/lib/prism/serialize.rb.erb b/prism/templates/lib/prism/serialize.rb.erb
index 1a723f64af..ac7ab4fff3 100644
--- a/prism/templates/lib/prism/serialize.rb.erb
+++ b/prism/templates/lib/prism/serialize.rb.erb
@@ -131,8 +131,8 @@ module Prism
         comments = load_comments
         magic_comments = Array.new(load_varuint) { MagicComment.new(load_location_object, load_location_object) }
         data_loc = load_optional_location_object
-        errors = Array.new(load_varuint) { ParseError.new(DIAGNOSTIC_TYPES[io.getbyte], load_embedded_string, load_location_object, load_error_level) }
-        warnings = Array.new(load_varuint) { ParseWarning.new(DIAGNOSTIC_TYPES[io.getbyte], load_embedded_string, load_location_object, load_warning_level) }
+        errors = Array.new(load_varuint) { ParseError.new(DIAGNOSTIC_TYPES[load_varuint], load_embedded_string, load_location_object, load_error_level) }
+        warnings = Array.new(load_varuint) { ParseWarning.new(DIAGNOSTIC_TYPES[load_varuint], load_embedded_string, load_location_object, load_warning_level) }
         [comments, magic_comments, data_loc, errors, warnings]
       end
 
@@ -296,9 +296,11 @@ module Prism
 
         case level
         when 0
-          :fatal
+          :syntax
         when 1
           :argument
+        when 2
+          :load
         else
           raise "Unknown level: #{level}"
         end
diff --git a/prism/templates/src/serialize.c.erb b/prism/templates/src/serialize.c.erb
index 94b976645d..97101e36d5 100644
--- a/prism/templates/src/serialize.c.erb
+++ b/prism/templates/src/serialize.c.erb
@@ -220,7 +220,7 @@ pm_serialize_data_loc(const pm_parser_t *parser, pm_buffer_t *buffer) {
 static void
 pm_serialize_diagnostic(pm_parser_t *parser, pm_diagnostic_t *diagnostic, pm_buffer_t *buffer) {
     // serialize the type
-    pm_buffer_append_byte(buffer, (uint8_t) diagnostic->diag_id);
+    pm_buffer_append_varuint(buffer, (uint32_t) diagnostic->diag_id);
 
     // serialize message
     size_t message_length = strlen(diagnostic->message);
diff --git a/test/prism/command_line_test.rb b/test/prism/command_line_test.rb
index 96ceb2da38..4b04c36f3a 100644
--- a/test/prism/command_line_test.rb
+++ b/test/prism/command_line_test.rb
@@ -88,5 +88,24 @@ module Prism
 
       assert_kind_of IntegerNode, result.value.statements.body.first
     end
+
+    def test_command_line_x_implicit_fail
+      result = Prism.parse(<<~RUBY)
+        #!/bin/bash
+        exit 1
+      RUBY
+
+      assert_equal 1, result.errors.length
+      assert_equal :load, result.errors.first.level
+    end
+
+    def test_command_line_x_explicit_fail
+      result = Prism.parse(<<~RUBY, command_line: "x")
+        exit 1
+      RUBY
+
+      assert_equal 1, result.errors.length
+      assert_equal :load, result.errors.first.level
+    end
   end
 end