diff --git a/PythonAPI/pycocotools/coco.py b/PythonAPI/pycocotools/coco.py
index e4182c74..4d1190ac 100644
--- a/PythonAPI/pycocotools/coco.py
+++ b/PythonAPI/pycocotools/coco.py
@@ -64,7 +64,8 @@
 
 
 def _isArrayLike(obj):
-    return hasattr(obj, '__iter__') and hasattr(obj, '__len__')
+    """Check if an object is array like (set, tuple, list, etc.)."""
+    return hasattr(obj, '__iter__') and hasattr(obj, '__len__') and not isinstance(obj, str)
 
 
 class COCO:
@@ -438,4 +439,4 @@ def annToMask(self, ann):
         """
         rle = self.annToRLE(ann)
         m = maskUtils.decode(rle)
-        return m
\ No newline at end of file
+        return m