diff --git a/sio/executors/checker.py b/sio/executors/checker.py
index d2300c6..98512ef 100644
--- a/sio/executors/checker.py
+++ b/sio/executors/checker.py
@@ -142,6 +142,8 @@ def run(environ, use_sandboxes=True):
 def output_to_fraction(output_str):
     if not output_str:
         return 100, 1
+    if isinstance(output_str, bytes):
+        output_str = output_str.decode('utf-8')
     try:
         frac = Fraction(output_str)
         return frac.numerator, frac.denominator
@@ -151,4 +153,6 @@ def output_to_fraction(output_str):
             % output_str
         )
     except ZeroDivisionError:
-        raise CheckerError('Zero division in checker output "%s"' % output_str)
\ No newline at end of file
+        raise CheckerError('Zero division in checker output "%s"' % output_str)
+    except TypeError:
+        raise CheckerError('Invalid checker output "%s"' % output_str)
\ No newline at end of file