diff --git a/survey_tools.py b/survey_tools.py
index 9803736..2bc2158 100644
--- a/survey_tools.py
+++ b/survey_tools.py
@@ -54,7 +54,7 @@ def call_lastools(tool_name, input, output=None, args=None, verbose=True):
 
     # Append additional lastools arguments, if provided
     if args:
-        cmd += args
+        cmd += [str(a) for a in args]
 
     process = subprocess.Popen(
         cmd,