diff --git a/read2tree/Mapper.py b/read2tree/Mapper.py
index f46baff..ea7b372 100644
--- a/read2tree/Mapper.py
+++ b/read2tree/Mapper.py
@@ -638,8 +638,10 @@ def _output_shell(self, line):
             return None
 
         (output, err) = shell_command.communicate()
-        self.logger.debug("Shell output: "+ str(output))
-        self.logger.debug("Shell err: " + str(err))
+        if output:
+            self.logger.debug("Shell output: "+ str(output))
+        if err:
+            self.logger.debug("Shell err: " + str(err))
         
         shell_command.wait()
         if shell_command.returncode != 0: