diff --git a/benchmark/problem_stats.py b/benchmark/problem_stats.py
index 7358a3827..004e4f24f 100755
--- a/benchmark/problem_stats.py
+++ b/benchmark/problem_stats.py
@@ -35,7 +35,7 @@ def load_results(dirname):
         error = False
         try:
             results = json.loads(fname.read_text())
-            error = 'testcase' not in results
+            error = "testcase" not in results
             if not error:
                 # Add language info to results
                 lang = fname.parts[-5]  # Get language from path