diff --git a/src/flake8/formatting/base.py b/src/flake8/formatting/base.py
index abad254..180380a 100644
--- a/src/flake8/formatting/base.py
+++ b/src/flake8/formatting/base.py
@@ -166,6 +166,8 @@ class BaseFormatter(object):
 
     def _write(self, output):
         """Handle logic of whether to use an output file or print()."""
+        if not isinstance(output, str):
+            output = output.encode('UTF-8')
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)
         if self.output_fd is None or self.options.tee: