diff --git a/examples/companion_radio/MyMesh.cpp b/examples/companion_radio/MyMesh.cpp
index 99a9aa30d6e..d74050be7e0 100644
--- a/examples/companion_radio/MyMesh.cpp
+++ b/examples/companion_radio/MyMesh.cpp
@@ -1175,7 +1175,9 @@ void MyMesh::handleCmdFrame(size_t len) {
                         ? ERR_CODE_NOT_FOUND
                         : ERR_CODE_UNSUPPORTED_CMD); // unknown recipient, or unsupported TXT_TYPE_*
     }
-  } else if (cmd_frame[0] == CMD_SEND_CHANNEL_TXT_MSG) { // send GroupChannel text msg
+  } else if (cmd_frame[0] == CMD_SEND_CHANNEL_TXT_MSG && len >= 7) { // send GroupChannel text msg
+    // 7 = code, txt_type, channel_idx and the 4 byte timestamp; the text may be empty.
+    // Without this the text length below is computed as len - 7 and turns negative.
     int i = 1;
     uint8_t txt_type = cmd_frame[i++]; // should be TXT_TYPE_PLAIN
     uint8_t channel_idx = cmd_frame[i++];
diff --git a/src/helpers/BaseChatMesh.cpp b/src/helpers/BaseChatMesh.cpp
index 28592415ca9..2637bc327ab 100644
--- a/src/helpers/BaseChatMesh.cpp
+++ b/src/helpers/BaseChatMesh.cpp
@@ -516,6 +516,11 @@ int  BaseChatMesh::sendCommandData(const ContactInfo& recipient, uint32_t timest
 }
 
 bool BaseChatMesh::sendGroupMessage(uint32_t timestamp, mesh::GroupChannel& channel, const char* sender_name, const char* text, int text_len) {
+  if (text_len < 0) {   // memcpy() below would read this as a size_t, so clamping cannot catch it
+    MESH_DEBUG_PRINTLN("sendGroupMessage: invalid negative text_len=%d", text_len);
+    return false;
+  }
+
   uint8_t temp[5+MAX_TEXT_LEN+32];
   memcpy(temp, &timestamp, 4);   // mostly an extra blob to help make packet_hash unique
   temp[4] = 0;  // TXT_TYPE_PLAIN