diff --git a/py/objstr.c b/py/objstr.c
index 07134687d1..4c09871b96 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -792,7 +792,7 @@ mp_obj_t str_format(uint n_args, const mp_obj_t *args) {
         if (arg_looks_integer(arg)) {
             switch (type) {
                 case 'b':
-                    pfenv_print_int(&pfenv_vstr, arg, 1, 2, 'a', flags, fill, width);
+                    pfenv_print_mp_int(&pfenv_vstr, arg, 1, 2, 'a', flags, fill, width);
                     continue;
 
                 case 'c':
@@ -805,7 +805,7 @@ mp_obj_t str_format(uint n_args, const mp_obj_t *args) {
                 case '\0':  // No explicit format type implies 'd'
                 case 'n':   // I don't think we support locales in uPy so use 'd'
                 case 'd':
-                    pfenv_print_int(&pfenv_vstr, arg, 1, 10, 'a', flags, fill, width);
+                    pfenv_print_mp_int(&pfenv_vstr, arg, 1, 10, 'a', flags, fill, width);
                     continue;
 
                 case 'o':
@@ -813,15 +813,15 @@ mp_obj_t str_format(uint n_args, const mp_obj_t *args) {
                         flags |= PF_FLAG_SHOW_OCTAL_LETTER;
                     }
 
-                    pfenv_print_int(&pfenv_vstr, arg, 1, 8, 'a', flags, fill, width);
+                    pfenv_print_mp_int(&pfenv_vstr, arg, 1, 8, 'a', flags, fill, width);
                     continue;
 
                 case 'x':
-                    pfenv_print_int(&pfenv_vstr, arg, 1, 16, 'a', flags, fill, width);
+                    pfenv_print_mp_int(&pfenv_vstr, arg, 1, 16, 'a', flags, fill, width);
                     continue;
 
                 case 'X':
-                    pfenv_print_int(&pfenv_vstr, arg, 1, 16, 'A', flags, fill, width);
+                    pfenv_print_mp_int(&pfenv_vstr, arg, 1, 16, 'A', flags, fill, width);
                     continue;
 
                 case 'e':
@@ -1042,7 +1042,7 @@ STATIC mp_obj_t str_modulo_format(mp_obj_t pattern, uint n_args, const mp_obj_t
             case 'd':
             case 'i':
             case 'u':
-                pfenv_print_int(&pfenv_vstr, arg_as_int(arg), 1, 10, 'a', flags, fill, width);
+                pfenv_print_mp_int(&pfenv_vstr, arg_as_int(arg), 1, 10, 'a', flags, fill, width);
                 break;
 
 #if MICROPY_ENABLE_FLOAT
@@ -1060,7 +1060,7 @@ STATIC mp_obj_t str_modulo_format(mp_obj_t pattern, uint n_args, const mp_obj_t
                 if (alt) {
                     flags |= (PF_FLAG_SHOW_PREFIX | PF_FLAG_SHOW_OCTAL_LETTER);
                 }
-                pfenv_print_int(&pfenv_vstr, arg_as_int(arg), 1, 8, 'a', flags, fill, width); 
+                pfenv_print_mp_int(&pfenv_vstr, arg_as_int(arg), 1, 8, 'a', flags, fill, width); 
                 break;
 
             case 'r':
@@ -1085,14 +1085,14 @@ STATIC mp_obj_t str_modulo_format(mp_obj_t pattern, uint n_args, const mp_obj_t
                 if (alt) {
                     flags |= PF_FLAG_SHOW_PREFIX;
                 }
-                pfenv_print_int(&pfenv_vstr, arg_as_int(arg), 1, 16, 'a', flags, fill, width);
+                pfenv_print_mp_int(&pfenv_vstr, arg_as_int(arg), 1, 16, 'a', flags, fill, width);
                 break;
 
             case 'X':
                 if (alt) {
                     flags |= PF_FLAG_SHOW_PREFIX;
                 }
-                pfenv_print_int(&pfenv_vstr, arg_as_int(arg), 1, 16, 'A', flags, fill, width);
+                pfenv_print_mp_int(&pfenv_vstr, arg_as_int(arg), 1, 16, 'A', flags, fill, width);
                 break;
 
             default:
diff --git a/py/pfenv.c b/py/pfenv.c
index ab45328ad7..b91390d6d9 100644
--- a/py/pfenv.c
+++ b/py/pfenv.c
@@ -77,11 +77,16 @@ int pfenv_print_strn(const pfenv_t *pfenv, const char *str, unsigned int len, in
     return len;
 }
 
+int pfenv_print_int(const pfenv_t *pfenv, int x, int sgn, int base, int base_char, int flags, char fill, int width) {
+    // XXX this really needs to be a dedicated function, since converting to a mp_int looses the MSB
+    return pfenv_print_mp_int(pfenv, MP_OBJ_NEW_SMALL_INT((machine_int_t)x), sgn, base, base_char, flags, fill, width);
+}
+
 // 32-bits is 10 digits, add 3 for commas, 1 for sign, 1 for terminating null
 // We can use 16 characters for 32-bit and 32 characters for 64-bit
 #define INT_BUF_SIZE (sizeof(machine_int_t) * 4)
 
-int pfenv_print_int(const pfenv_t *pfenv, mp_obj_t x, int sgn, int base, int base_char, int flags, char fill, int width) {
+int pfenv_print_mp_int(const pfenv_t *pfenv, mp_obj_t x, int sgn, int base, int base_char, int flags, char fill, int width) {
     if (!MP_OBJ_IS_INT(x)) {
         // This will convert booleans to int, or raise an error for
         // non-integer types.
diff --git a/py/pfenv.h b/py/pfenv.h
index 32ecc159ff..6240b47acb 100644
--- a/py/pfenv.h
+++ b/py/pfenv.h
@@ -18,7 +18,8 @@ typedef struct _pfenv_t {
 void pfenv_vstr_add_strn(void *data, const char *str, unsigned int len);
 
 int pfenv_print_strn(const pfenv_t *pfenv, const char *str, unsigned int len, int flags, char fill, int width);
-int pfenv_print_int(const pfenv_t *pfenv, mp_obj_t x, int sgn, int base, int base_char, int flags, char fill, int width);
+int pfenv_print_int(const pfenv_t *pfenv, int x, int sgn, int base, int base_char, int flags, char fill, int width);
+int pfenv_print_mp_int(const pfenv_t *pfenv, mp_obj_t x, int sgn, int base, int base_char, int flags, char fill, int width);
 #if MICROPY_ENABLE_FLOAT
 int pfenv_print_float(const pfenv_t *pfenv, mp_float_t f, char fmt, int flags, char fill, int width, int prec);
 #endif