diff --git a/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip32.h b/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip32.h
index 1542b2173..b4f94a29f 100644
--- a/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip32.h
+++ b/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip32.h
@@ -191,7 +191,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_HDNode_derive_path_obj, mod_tr
 
 STATIC mp_obj_t serialize_public_private(mp_obj_t self, bool use_public, uint32_t version) {
     mp_obj_HDNode_t *o = MP_OBJ_TO_PTR(self);
-    char xpub[XPUB_MAXLEN];
+    char xpub[XPUB_MAXLEN] = {0};
     int written;
     if (use_public) {
         hdnode_fill_public_key(&o->hdnode);
@@ -202,7 +202,7 @@ STATIC mp_obj_t serialize_public_private(mp_obj_t self, bool use_public, uint32_
     if (written <= 0) {
         mp_raise_ValueError("Failed to serialize");
     }
-    return mp_obj_new_str(xpub, written - 1, false);  // written includes 0 at the end
+    return mp_obj_new_str_copy(&mp_type_str, (const uint8_t *)xpub, written - 1);  // written includes 0 at the end
 }
 
 /// def serialize_public(self, version: int) -> str:
@@ -309,9 +309,9 @@ STATIC mp_obj_t mod_trezorcrypto_HDNode_address(mp_obj_t self, mp_obj_t version)
 
     uint32_t v = trezor_obj_get_uint(version);
 
-    char address[ADDRESS_MAXLEN];
+    char address[ADDRESS_MAXLEN] = {0};
     hdnode_get_address(&o->hdnode, v, address, ADDRESS_MAXLEN);
-    return mp_obj_new_str(address, strlen(address), false);
+    return mp_obj_new_str_copy(&mp_type_str, (const uint8_t *)address, strlen(address));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_HDNode_address_obj, mod_trezorcrypto_HDNode_address);
 
@@ -324,11 +324,11 @@ STATIC mp_obj_t mod_trezorcrypto_HDNode_nem_address(mp_obj_t self, mp_obj_t netw
 
     uint8_t n = trezor_obj_get_uint8(network);
 
-    char address[NEM_ADDRESS_SIZE + 1]; // + 1 for the 0 byte
+    char address[NEM_ADDRESS_SIZE + 1] = {0}; // + 1 for the 0 byte
     if (!hdnode_get_nem_address(&o->hdnode, n, address)) {
         mp_raise_ValueError("Failed to compute a NEM address");
     }
-    return mp_obj_new_str_of_type(&mp_type_str, (const uint8_t *)address, strlen(address));
+    return mp_obj_new_str_copy(&mp_type_str, (const uint8_t *)address, strlen(address));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorcrypto_HDNode_nem_address_obj, mod_trezorcrypto_HDNode_nem_address);
 
diff --git a/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip39.h b/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip39.h
index e9c5dc191..e6f412f3d 100644
--- a/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip39.h
+++ b/embed/extmod/modtrezorcrypto/modtrezorcrypto-bip39.h
@@ -34,7 +34,7 @@ STATIC mp_obj_t mod_trezorcrypto_bip39_find_word(mp_obj_t prefix)
     }
     for (const char * const *w = mnemonic_wordlist(); *w != 0; w++) {
         if (strncmp(*w, pfx.buf, pfx.len) == 0) {
-            return mp_obj_new_str(*w, strlen(*w), false);
+            return mp_obj_new_str(*w, strlen(*w));
         }
     }
     return mp_const_none;
@@ -78,7 +78,7 @@ STATIC mp_obj_t mod_trezorcrypto_bip39_generate(mp_obj_t strength) {
         mp_raise_ValueError("Invalid bit strength (only 128, 160, 192, 224 and 256 values are allowed)");
     }
     const char *mnemo = mnemonic_generate(bits);
-    return mp_obj_new_str(mnemo, strlen(mnemo), false);
+    return mp_obj_new_str_copy(&mp_type_str, (const uint8_t *)mnemo, strlen(mnemo));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_trezorcrypto_bip39_generate_obj, mod_trezorcrypto_bip39_generate);
 
@@ -93,7 +93,7 @@ STATIC mp_obj_t mod_trezorcrypto_bip39_from_data(mp_obj_t data) {
         mp_raise_ValueError("Invalid data length (only 16, 20, 24, 28 and 32 bytes are allowed)");
     }
     const char *mnemo = mnemonic_from_data(bin.buf, bin.len);
-    return mp_obj_new_str(mnemo, strlen(mnemo), false);
+    return mp_obj_new_str_copy(&mp_type_str, (const uint8_t *)mnemo, strlen(mnemo));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_trezorcrypto_bip39_from_data_obj, mod_trezorcrypto_bip39_from_data);
 
diff --git a/embed/extmod/modtrezorutils/modtrezorutils.c b/embed/extmod/modtrezorutils/modtrezorutils.c
index ca33c3e26..13f752553 100644
--- a/embed/extmod/modtrezorutils/modtrezorutils.c
+++ b/embed/extmod/modtrezorutils/modtrezorutils.c
@@ -125,7 +125,7 @@ STATIC mp_obj_t mod_trezorutils_symbol(mp_obj_t name) {
     if (0 == strncmp(str.buf, "GITREV", str.len)) {
 #define XSTR(s) STR(s)
 #define STR(s) #s
-        return mp_obj_new_str(XSTR(GITREV), strlen(XSTR(GITREV)), false);
+        return mp_obj_new_str(XSTR(GITREV), strlen(XSTR(GITREV)));
     }
     if (0 == strncmp(str.buf, "VERSION_MAJOR", str.len)) {
         return mp_obj_new_int(VERSION_MAJOR);
@@ -151,7 +151,7 @@ STATIC mp_obj_t mod_trezorutils_model(void) {
 #elif defined TREZOR_MODEL_EMU
     model = "EMU";
 #endif
-    return model ? mp_obj_new_str(model, strlen(model), false) : mp_const_none;
+    return model ? mp_obj_new_str(model, strlen(model)) : mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(mod_trezorutils_model_obj, mod_trezorutils_model);
 
diff --git a/embed/unix/main.c b/embed/unix/main.c
index 671d978a5..c1c51da07 100644
--- a/embed/unix/main.c
+++ b/embed/unix/main.c
@@ -537,7 +537,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
                     return usage(argv);
                 }
                 mp_obj_t import_args[4];
-                import_args[0] = mp_obj_new_str(argv[a + 1], strlen(argv[a + 1]), false);
+                import_args[0] = mp_obj_new_str(argv[a + 1], strlen(argv[a + 1]));
                 import_args[1] = import_args[2] = mp_const_none;
                 // Ask __import__ to handle imported module specially - set its __name__
                 // to __main__, and also return this leaf module, not top-level package