diff --git a/mongoose.c b/mongoose.c
index 3f4d0b0b..adee0e4c 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -4368,7 +4368,7 @@ static void mg_iotest(struct mg_mgr *mgr, int ms) {
   nfds_t n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) n++;
   struct pollfd *fds = (struct pollfd *) alloca(n * sizeof(fds[0]));
-  if (n > 0) memset(fds, 0, sizeof(n * sizeof(fds[0])));
+  memset(fds, 0, n * sizeof(fds[0]));
   n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) {
     c->is_readable = c->is_writable = 0;
@@ -4390,7 +4390,7 @@ static void mg_iotest(struct mg_mgr *mgr, int ms) {
 #if MG_ARCH == MG_ARCH_WIN32
     if (n == 0) Sleep(ms);  // On Windows, poll fails if no sockets
 #endif
-    if (n > 0) memset(fds, 0, sizeof(n * sizeof(fds[0])));
+    memset(fds, 0, n * sizeof(fds[0]));
   }
   n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) {
@@ -5409,7 +5409,7 @@ 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;
+    uint8_t c = ((uint8_t *) buf)[i] % 62U;
     buf[i] = i == len - 1 ? (char) '\0'            // 0-terminate last byte
              : c < 26     ? (char) ('a' + c)       // lowercase
              : c < 52     ? (char) ('A' + c - 26)  // uppercase
diff --git a/src/sock.c b/src/sock.c
index ba09dd38..21c859c3 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -522,7 +522,7 @@ static void mg_iotest(struct mg_mgr *mgr, int ms) {
   nfds_t n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) n++;
   struct pollfd *fds = (struct pollfd *) alloca(n * sizeof(fds[0]));
-  if (n > 0) memset(fds, 0, sizeof(n * sizeof(fds[0])));
+  memset(fds, 0, n * sizeof(fds[0]));
   n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) {
     c->is_readable = c->is_writable = 0;
@@ -544,7 +544,7 @@ static void mg_iotest(struct mg_mgr *mgr, int ms) {
 #if MG_ARCH == MG_ARCH_WIN32
     if (n == 0) Sleep(ms);  // On Windows, poll fails if no sockets
 #endif
-    if (n > 0) memset(fds, 0, sizeof(n * sizeof(fds[0])));
+    memset(fds, 0, n * sizeof(fds[0]));
   }
   n = 0;
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) {
diff --git a/src/util.c b/src/util.c
index 0d34b94c..4725edd6 100644
--- a/src/util.c
+++ b/src/util.c
@@ -25,7 +25,7 @@ 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;
+    uint8_t c = ((uint8_t *) buf)[i] % 62U;
     buf[i] = i == len - 1 ? (char) '\0'            // 0-terminate last byte
              : c < 26     ? (char) ('a' + c)       // lowercase
              : c < 52     ? (char) ('A' + c - 26)  // uppercase
diff --git a/test/unit_test.c b/test/unit_test.c
index 49e261b0..d77dc579 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -1758,6 +1758,19 @@ static void test_util(void) {
   ASSERT(mg_tou64(mg_str("")) == 0);
   ASSERT(mg_tou64(mg_str("-")) == 0);
   ASSERT(mg_tou64(mg_str("18446744073709551615")) == 18446744073709551615U);
+
+  {
+    size_t i;
+    memset(buf, ' ', sizeof(buf));
+    mg_random_str(buf, 0);
+    ASSERT(buf[0] == ' ');
+    mg_random_str(buf, 1);
+    ASSERT(buf[0] == '\0');
+    ASSERT(buf[1] == ' ');
+    mg_random_str(buf, sizeof(buf));
+    ASSERT(buf[sizeof(buf) - 1] == '\0');
+    for (i = 0; i < sizeof(buf) - 1; i++) ASSERT(isalnum((uint8_t) buf[i]));
+  }
 }
 
 static void test_crc32(void) {