diff --git a/files/usr/local/bin/rssi_monitor b/files/usr/local/bin/rssi_monitor
index c09d180c..d39f4d62 100755
--- a/files/usr/local/bin/rssi_monitor
+++ b/files/usr/local/bin/rssi_monitor
@@ -293,7 +293,7 @@ undef $/;
 $^I = "";
 while (<>) 
 {
-  substr($_, 0, $MAXSIZE - length) = "";
+  substr($_, 0, length() - $MAXSIZE) = "";
   s/.*\n//;
   print;
 }