diff --git a/tools/harness-thci/OpenThread_WpanCtl.py b/tools/harness-thci/OpenThread_WpanCtl.py
index b6bc101d6..2a02ed7f1 100644
--- a/tools/harness-thci/OpenThread_WpanCtl.py
+++ b/tools/harness-thci/OpenThread_WpanCtl.py
@@ -2048,7 +2048,7 @@ class OpenThread_WpanCtl(IThci):
         # long timeout value to avoid automatic joiner removal (in seconds)
         timeout = 500
 
-        cmd = self.wpan_cmd_prefix + 'commissioner joiner-add %s %s %s' % (eui64, str(timeout), strPSKd)
+        cmd = self.wpan_cmd_prefix + 'commissioner joiner-add "%s" %s %s' % (eui64, str(timeout), strPSKd)
         print(cmd)
         if not self.isActiveCommissioner:
             self.startCollapsedCommissioner()