diff --git a/lib/lyx2lyx/parser_tools.py b/lib/lyx2lyx/parser_tools.py
index 486cfac9e1..78459d21d9 100644
--- a/lib/lyx2lyx/parser_tools.py
+++ b/lib/lyx2lyx/parser_tools.py
@@ -176,6 +176,8 @@ def find_beginning_of(lines, i, start_token, end_token):
     count = 1
     while i > 0:
         i = find_tokens_backwards(lines, [start_token, end_token], i-1)
+        if i == -1:
+            return -1
         if check_token(lines[i], end_token):
             count = count+1
         else:
@@ -190,6 +192,8 @@ def find_end_of(lines, i, start_token, end_token):
     n = len(lines)
     while i < n:
         i = find_tokens(lines, [end_token, start_token], i+1)
+        if i == -1:
+            return -1
         if check_token(lines[i], start_token):
             count = count+1
         else: