diff --git a/scripts/xsslint/xsslint/linters.py b/scripts/xsslint/xsslint/linters.py
index 5b1a2aaa40..a183cc028a 100644
--- a/scripts/xsslint/xsslint/linters.py
+++ b/scripts/xsslint/xsslint/linters.py
@@ -1090,7 +1090,7 @@ class MakoTemplateLinter(BaseLinter):
         javascript_start_index = None
         for context in contexts:
             if context['type'] == 'javascript':
-                if javascript_start_index < 0:
+                if javascript_start_index is None:
                     javascript_start_index = context['index']
             else:
                 if javascript_start_index is not None:
@@ -1349,7 +1349,8 @@ class MakoTemplateLinter(BaseLinter):
         while True:
             parse_string = ParseString(scrubbed_lines, start_index, len(scrubbed_lines))
             # check for validly parsed string
-            if 0 <= parse_string.start_index < parse_string.end_index:
+            if (parse_string.start_index is not None and parse_string.end_index is not None) \
+                    and (0 <= parse_string.start_index < parse_string.end_index):
                 # check if expression is contained in the given string
                 if parse_string.start_index < adjusted_start_index < parse_string.end_index:
                     return parse_string