diff --git a/src/axp.ino b/src/axp.ino
index e18d700..cda5357 100644
--- a/src/axp.ino
+++ b/src/axp.ino
@@ -17,9 +17,9 @@ static AXP20X_Class axp;
 static uint32_t next_axp_log_millis;
 static uint32_t next_axp_report_millis;
 
-static char str_axp_temp[10];
-static char str_axp_bus[20];
-static char str_axp_bat[60];
+static char str_axp_temp[10] = {'\0'};
+static char str_axp_bus[20] = {'\0'};
+static char str_axp_bat[60] = {'\0'};
 
 // ----------------------------------------------------------------------------
 bool axp_setup() {
@@ -113,6 +113,8 @@ void axp_update_data() {
 
 // ----------------------------------------------------------------------------
 char *axp_update_str(const char *fmt) {
+    if (str_axp_temp[0] == '\0' || str_axp_bus[0] == '\0' || str_axp_bat[0] == '\0') return NULL;
+
     static char str[sizeof(str_axp_temp) + sizeof(str_axp_bus) + sizeof(str_axp_bat) + 10];
     snprintf(str, sizeof(str), fmt, str_axp_temp, str_axp_bus, str_axp_bat);
     return str;
diff --git a/src/gps.ino b/src/gps.ino
index 025206d..a8a6489 100644
--- a/src/gps.ino
+++ b/src/gps.ino
@@ -25,9 +25,9 @@ static TinyGPSPlus gps;
 static uint32_t next_gps_stamp_millis;
 static uint32_t next_gps_report_millis;
 
-static char str_gps_datetime[20];
-static char str_gps_loc[32];
-static char str_gps_quality[10];
+static char str_gps_datetime[20] = {'\0'};
+static char str_gps_loc[32] = {'\0'};
+static char str_gps_quality[10] = {'\0'};
 
 // ----------------------------------------------------------------------------
 void gps_setup(bool do_axp_exist) {
@@ -91,6 +91,8 @@ void gps_update_data() {
 
 // ----------------------------------------------------------------------------
 char *gps_update_str(const char *fmt) {
+    if (str_gps_datetime[0] == '\0' || str_gps_loc[0] == '\0' || str_gps_quality[0] == '\0') return NULL;
+
     static char str[sizeof(str_gps_datetime) + sizeof(str_gps_loc) + sizeof(str_gps_quality) + 10];
     snprintf(str, sizeof(str), fmt, str_gps_datetime, str_gps_loc, str_gps_quality);
     return str;
diff --git a/src/lora.ino b/src/lora.ino
index 99a0c18..7630188 100644
--- a/src/lora.ino
+++ b/src/lora.ino
@@ -106,6 +106,10 @@ bool report_status_to(Address sink) {
 // ----------------------------------------------------------------------------
 bool report_gps_to(Address sink) {
     char *str = gps_update_str("@GPS %s\n%s\n%s\n");
+    if (str == NULL) {
+        term_println("[LORA] report_gps_to(): gps_update_str() return NULL!");
+        return false;
+    }
     uint8_t cnt = strlen(str);
 
     term_printf("[LORA] Report GPS node %d to %d, %d bytes:", getAddress(), sink, cnt);
@@ -118,6 +122,10 @@ bool report_gps_to(Address sink) {
 // ----------------------------------------------------------------------------
 bool report_axp_to(Address sink) {
     char *str = axp_update_str("@AXP %s\n%s\n%s\n");
+    if (str == NULL) {
+        term_println("[LORA] report_axp_to(): axp_update_str() return NULL!");
+        return false;
+    }
     uint8_t cnt = strlen(str);
 
     term_printf("[LORA] Report AXP node %d to %d, %d bytes:", getAddress(), sink, cnt);