diff --git a/tests/z_msgcodec_test.c b/tests/z_msgcodec_test.c
index dcf051d41..b56b40a9f 100644
--- a/tests/z_msgcodec_test.c
+++ b/tests/z_msgcodec_test.c
@@ -184,7 +184,9 @@ _z_bytes_t gen_bytes(size_t len) {
     if (len == 0) return arr;
 
     arr.start = (uint8_t *)z_malloc(sizeof(uint8_t) * len);
-    for (_z_zint_t i = 0; i < len; i++) ((uint8_t *)arr.start)[i] = gen_uint8();
+    for (_z_zint_t i = 0; i < len; i++) {
+        ((uint8_t *)arr.start)[i] = gen_uint8() & 0b01111111;
+    }
 
     return arr;
 }