diff --git a/devices/m5310a/m5310a.c b/devices/m5310a/m5310a.c
index 650bac58..1a82cf61 100644
--- a/devices/m5310a/m5310a.c
+++ b/devices/m5310a/m5310a.c
@@ -262,6 +262,10 @@ static int m5310a_signal_quality_check(void)
     }
 
     str = strstr(echo.buffer, "+CSQ:");
+    if (!str) 
+    {
+        return -1;
+    }
     sscanf(str, "+CSQ:%d,%d", &rssi, &ber);
     if (rssi == 99) {
         return -1;