diff --git a/CHANGELOG_LATEST.md b/CHANGELOG_LATEST.md
index ed105de5..4840d8c7 100644
--- a/CHANGELOG_LATEST.md
+++ b/CHANGELOG_LATEST.md
@@ -3,6 +3,7 @@
 ### Added
 
 ### Fixed
+- telegrams matched to masterthermostat 0x18
 
 ### Changed
 
diff --git a/src/emsesp.cpp b/src/emsesp.cpp
index 882ad53e..d7d515a0 100644
--- a/src/emsesp.cpp
+++ b/src/emsesp.cpp
@@ -130,7 +130,7 @@ uint8_t EMSESP::check_master_device(const uint8_t device_id, const uint16_t type
             }
         }
         // look for ids that are only handled by master
-        for (uint8_t i = 0; i < sizeof(master_ids); i++) {
+        for (uint8_t i = 0; i < sizeof(master_ids) / 2; i++) {
             if (type_id == master_ids[i]) {
                 return 0x18;
             }