diff --git a/pdfindex/utils.py b/pdfindex/utils.py
index 3267ab3..23dc3f6 100644
--- a/pdfindex/utils.py
+++ b/pdfindex/utils.py
@@ -5,8 +5,8 @@
 
 def flatten(l):
     for el in l:
-        if isinstance(el, collections.Iterable) and not isinstance(el, str):
+        if isinstance(el, collections.abc.Iterable) and not isinstance(el, str):
             for sub in flatten(el):
                 yield sub
         else:
-            yield el
\ No newline at end of file
+            yield el