diff --git a/src/main.cpp b/src/main.cpp
index 69ed3b9..a69f228 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -180,7 +180,7 @@
 static int handle_msg(handle_t chan, uint32_t req_len, struct chan_ctx* ctx) {
     int rc;
     uint8_t msg[CONFIRMATIONUI_MAX_MSG_SIZE];
-    uint32_t resp_len = sizeof(msg);
+    uint32_t resp_len = ctx->shm_len;
     struct confirmationui_hdr hdr;
     struct confirmationui_msg_args args;