diff --git a/fw/Core/Src/main.c b/fw/Core/Src/main.c
index 6f8ed44..8aeaecc 100644
--- a/fw/Core/Src/main.c
+++ b/fw/Core/Src/main.c
@@ -123,6 +123,7 @@ int8_t modbus_slave_callback(modbus_transaction_t *transaction)
 		// readout of all measured variables
 		if (transaction->register_number == 30010) { // register number 30010 (randomly selected)
 			transaction->input_registers[0] = CO2;
+			return MODBUS_OK;
 		}
 	}
 	return MODBUS_ERROR_FUNCTION_NOT_IMPLEMENTED; /* nothing implemented yet! TODO */