diff --git a/ports/nrf/common-hal/os/__init__.c b/ports/nrf/common-hal/os/__init__.c
index e38226f94e..b2ad00a5ca 100644
--- a/ports/nrf/common-hal/os/__init__.c
+++ b/ports/nrf/common-hal/os/__init__.c
@@ -66,8 +66,24 @@ bool common_hal_os_urandom(uint8_t *buffer, uint32_t length) {
     uint8_t sd_en = 0;
     (void) sd_softdevice_is_enabled(&sd_en);
 
-    if (sd_en)
-        return NRF_SUCCESS == sd_rand_application_vector_get(buffer, length);
+    if (sd_en) {
+        while (length != 0) {
+            uint8_t available = 0;
+            sd_rand_application_bytes_available_get(&available);
+            if (available) {
+                uint32_t request = MIN(length, available);
+                uint32_t result = sd_rand_application_vector_get(buffer, request);
+                if (result != NRF_SUCCESS) {
+                    return false;
+                }
+                buffer += request;
+                length -= request;
+            } else {
+                RUN_BACKGROUND_TASKS;
+            }
+        }
+        return true;
+    }
 #endif
 
     nrf_rng_event_clear(NRF_RNG, NRF_RNG_EVENT_VALRDY);
diff --git a/py/vstr.c b/py/vstr.c
index 869b278057..91cd7f584f 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -50,6 +50,9 @@ void vstr_init(vstr_t *vstr, size_t alloc) {
 // Init the vstr so it allocs exactly enough ram to hold a null-terminated
 // string of the given length, and set the length.
 void vstr_init_len(vstr_t *vstr, size_t len) {
+    if(len == SIZE_MAX) {
+        m_malloc_fail(len);
+    }
     vstr_init(vstr, len + 1);
     vstr->len = len;
 }
diff --git a/shared-bindings/os/__init__.c b/shared-bindings/os/__init__.c
index 55ebd6f59f..c2b63bbcee 100644
--- a/shared-bindings/os/__init__.c
+++ b/shared-bindings/os/__init__.c
@@ -33,6 +33,7 @@
 #include "lib/oofatfs/diskio.h"
 #include "py/mpstate.h"
 #include "py/obj.h"
+#include "py/objstr.h"
 #include "py/runtime.h"
 #include "shared-bindings/os/__init__.h"
 
@@ -195,11 +196,11 @@ MP_DEFINE_CONST_FUN_OBJ_0(os_sync_obj, os_sync);
 //|
 STATIC mp_obj_t os_urandom(mp_obj_t size_in) {
     mp_int_t size = mp_obj_get_int(size_in);
-    uint8_t tmp[size];
-    if (!common_hal_os_urandom(tmp, size)) {
+    mp_obj_str_t *result = MP_OBJ_TO_PTR(mp_obj_new_bytes_of_zeros(size));
+    if (!common_hal_os_urandom((uint8_t*) result->data, size)) {
         mp_raise_NotImplementedError(translate("No hardware random available"));
     }
-    return mp_obj_new_bytes(tmp, size);
+    return result;
 }
 MP_DEFINE_CONST_FUN_OBJ_1(os_urandom_obj, os_urandom);