--- a/dom/media/webrtc/RTCCertificate.cpp
+++ b/dom/media/webrtc/RTCCertificate.cpp
@@ -70,17 +70,17 @@ private:
                                              sizeof(randomName));
     if (rv != SECSuccess) {
       return nullptr;
     }
 
     char buf[sizeof(randomName) * 2 + 4];
     PL_strncpy(buf, "CN=", 3);
     for (size_t i = 0; i < sizeof(randomName); ++i) {
-      snprintf(&buf[i * 2 + 3], 2, "%.2x", randomName[i]);
+      snprintf(&buf[i * 2 + 3], 3, "%.2x", randomName[i]);
     }
     buf[sizeof(buf) - 1] = '\0';
 
     return CERT_AsciiToName(buf);
   }
 
   nsresult GenerateCertificate()
   {