diff --git a/tools/toolchains/__init__.py b/tools/toolchains/__init__.py
index fc08727593..4d10d787f1 100644
--- a/tools/toolchains/__init__.py
+++ b/tools/toolchains/__init__.py
@@ -958,7 +958,7 @@ class mbedToolchain:
 
     def compile_output(self, output=[]):
         _rc = output[0]
-        _stderr = output[1]
+        _stderr = output[1].decode("utf-8")
         command = output[2]
 
         # Parse output for Warnings and Errors