diff --git a/direct/src/stdpy/file.py b/direct/src/stdpy/file.py
index b911fbba84..1665648bf9 100644
--- a/direct/src/stdpy/file.py
+++ b/direct/src/stdpy/file.py
@@ -57,6 +57,7 @@ class file:
             binary = False
             if 'b' in mode:
                 # Strip 'b'.  This means a binary file.
+                i = mode.index('b')
                 mode = mode[:i] + mode[i + 1:]
                 binary = True