diff --git a/bbn_wave_freq_m5atomS3/NmeaChecksum.h b/bbn_wave_freq_m5atomS3/NmeaChecksum.h
index ebf56e5..a0a0d39 100644
--- a/bbn_wave_freq_m5atomS3/NmeaChecksum.h
+++ b/bbn_wave_freq_m5atomS3/NmeaChecksum.h
@@ -11,7 +11,7 @@ int nmea0183_checksum(char *nmea_data_before_asterisk) {
     int crc = 0;
     int i;
     // the first $ sign 
-    for (i = 1; i < strlen(nmea_data); i ++) {
+    for (i = 1; i < strlen(nmea_data_before_asterisk); i ++) {
         crc ^= nmea_data[i];
     }
     return crc;