diff --git a/docs/README.md b/docs/README.md
index 33798d4c..a4729eaf 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -3239,6 +3239,27 @@ char buf[10];
 mg_random(buf, sizeof(buf)); // `buf` is now random bytes
 ```
 
+### mg\_random\_str()
+
+```c
+char *mg_random_str(char *buf, size_t len);
+```
+
+Fill in buffer `buf`, `len` with random alphanumeric characters: `a-zA-Z0-9`.
+A buffer is zero-terminated.
+
+Parameters:
+- `buf` - a pointer to a buffer
+- `len` - a buffer size
+
+Return value: `buf` value.
+
+Usage example:
+```c
+char buf[10];
+printf("Random: %s\n", mg_random_str(buf, sizeof(buf)));
+```
+
 ### mg\_ntohs()
 
 ```c
diff --git a/mongoose.c b/mongoose.c
index cca8d0de..dadab9ab 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -5407,6 +5407,19 @@ void mg_random(void *buf, size_t len) {
 }
 #endif
 
+char *mg_random_str(char *buf, size_t len) {
+  size_t i;
+  mg_random(buf, len);
+  for (i = 0; i < len; i++) {
+    uint8_t c = ((uint8_t *) buf)[i] % 62;
+    buf[i] = i == len - 1 ? '\0'                   // 0-terminate last byte
+             : c < 26     ? (char) ('a' + c)       // lowercase
+             : c < 52     ? (char) ('A' + c - 26)  // uppercase
+                          : (char) ('0' + c - 52);     // numeric
+  }
+  return buf;
+}
+
 uint32_t mg_ntohl(uint32_t net) {
   uint8_t data[4] = {0, 0, 0, 0};
   memcpy(&data, &net, sizeof(data));
diff --git a/mongoose.h b/mongoose.h
index bf6699ce..fc00967b 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -858,6 +858,7 @@ bool mg_file_printf(struct mg_fs *fs, const char *path, const char *fmt, ...);
 
 
 void mg_random(void *buf, size_t len);
+char *mg_random_str(char *buf, size_t len);
 uint16_t mg_ntohs(uint16_t net);
 uint32_t mg_ntohl(uint32_t net);
 uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len);
diff --git a/src/util.c b/src/util.c
index 4dce7ed4..06b3fd7c 100644
--- a/src/util.c
+++ b/src/util.c
@@ -21,6 +21,19 @@ void mg_random(void *buf, size_t len) {
 }
 #endif
 
+char *mg_random_str(char *buf, size_t len) {
+  size_t i;
+  mg_random(buf, len);
+  for (i = 0; i < len; i++) {
+    uint8_t c = ((uint8_t *) buf)[i] % 62;
+    buf[i] = i == len - 1 ? '\0'                   // 0-terminate last byte
+             : c < 26     ? (char) ('a' + c)       // lowercase
+             : c < 52     ? (char) ('A' + c - 26)  // uppercase
+                          : (char) ('0' + c - 52);     // numeric
+  }
+  return buf;
+}
+
 uint32_t mg_ntohl(uint32_t net) {
   uint8_t data[4] = {0, 0, 0, 0};
   memcpy(&data, &net, sizeof(data));
diff --git a/src/util.h b/src/util.h
index 5bf4b070..6b4a0860 100644
--- a/src/util.h
+++ b/src/util.h
@@ -5,6 +5,7 @@
 #include "str.h"
 
 void mg_random(void *buf, size_t len);
+char *mg_random_str(char *buf, size_t len);
 uint16_t mg_ntohs(uint16_t net);
 uint32_t mg_ntohl(uint32_t net);
 uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len);
diff --git a/test/unit_test.c b/test/unit_test.c
index 773853ba..49e261b0 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -373,7 +373,7 @@ static void mqtt_cb(struct mg_connection *c, int ev, void *evd, void *fnd) {
 }
 
 static void test_mqtt_ver(uint8_t mqtt_version) {
-  char buf[50] = {0};
+  char buf[50] = {0}, client_id[16], will_topic[16];
   struct mqtt_data test_data = {buf, 0, 0};
   struct mg_mgr mgr;
   struct mg_str topic = mg_str("x/f12"), data = mg_str("hi");
@@ -410,9 +410,9 @@ static void test_mqtt_ver(uint8_t mqtt_version) {
   opts.will_retain = true;
   opts.keepalive = 20;
   opts.version = mqtt_version;
-  opts.will_topic = mg_str("mg_will_topic");
+  opts.will_topic = mg_str(mg_random_str(will_topic, sizeof(will_topic)));
   opts.will_message = mg_str("mg_will_messsage");
-  opts.client_id = mg_str("mg_unit_test");
+  opts.client_id = mg_str(mg_random_str(client_id, sizeof(client_id)));
   c = mg_mqtt_connect(&mgr, url, &opts, mqtt_cb, &test_data);
   for (i = 0; i < 300 && buf[0] == 0; i++) mg_mgr_poll(&mgr, 10);
   if (buf[0] != 'X') MG_INFO(("[%s]", buf));