diff --git a/mqtt2influxdb/expr.py b/mqtt2influxdb/expr.py
index 76b97fc..f1ad1e0 100644
--- a/mqtt2influxdb/expr.py
+++ b/mqtt2influxdb/expr.py
@@ -11,7 +11,7 @@ def jsonpath_to_variable(p):
 def variable_to_jsonpath(p):
     """Converts a expression variable into a valid JSON path starting with $."""
     # replace JSON_ with $ and _ with .
-    return p.var.replace('JSON_', '$').replace('_', '.')
+    return p.replace('JSON_', '$').replace('_', '.')
 
 
 def parse_expression(txt):