diff --git a/strings/min_window_substring.py b/strings/min_window_substring.py
index d19ff433c..d241bdc3f 100644
--- a/strings/min_window_substring.py
+++ b/strings/min_window_substring.py
@@ -28,7 +28,7 @@ def min_window(search_str: str, target_letters: str) -> str:
 
     # Return if not possible due to string lengths.
     if search_len < target_count:
-        return ''
+        return ""
 
     # Build dictionary with counts for each letter in target_letters
     char_dict = {}
@@ -82,4 +82,4 @@ def min_window(search_str: str, target_letters: str) -> str:
     if exists:
         return search_str[min_window[0] : min_window[1]]
     else:
-        return ''
+        return ""