diff --git a/sonoff/support.ino b/sonoff/support.ino
index c5e5dcc42..8b3cbe58a 100644
--- a/sonoff/support.ino
+++ b/sonoff/support.ino
@@ -154,7 +154,7 @@ char* subStr(char* dest, char* str, const char *delim, int index)
   return sub;
 }
 
-double CharToDouble(char *str)
+double CharToDouble(const char *str)
 {
   // simple ascii to double, because atof or strtod are too large
   char strbuf[24];