diff --git a/libc/fmt/fmt.c b/libc/fmt/fmt.c
index 99b8e8afd..be716b25f 100644
--- a/libc/fmt/fmt.c
+++ b/libc/fmt/fmt.c
@@ -317,6 +317,7 @@ _Hide int __fmt(void *fn, void *arg, const char *format, va_list va) {
         goto FormatNumber;
       case 'b':
         log2base = 1;
+        alphabet = "0123456789abcdefpb";
         goto FormatNumber;
       case 'o':
         log2base = 3;
diff --git a/libc/fmt/ntoa.c b/libc/fmt/ntoa.c
index 8b1878df0..774cbb475 100644
--- a/libc/fmt/ntoa.c
+++ b/libc/fmt/ntoa.c
@@ -31,7 +31,7 @@ uint128_t __udivmodti4(uint128_t, uint128_t, uint128_t *);
 static int __fmt_ntoa_format(int out(const char *, void *, size_t), void *arg,
                              char *buf, unsigned len, bool negative,
                              unsigned log2base, unsigned prec, unsigned width,
-                             unsigned char flags) {
+                             unsigned char flags, const char *alphabet) {
   unsigned i;
   /* pad leading zeros */
   if (width && (flags & FLAGS_ZEROPAD) &&
@@ -53,10 +53,9 @@ static int __fmt_ntoa_format(int out(const char *, void *, size_t), void *arg,
         len--;
       }
     }
-    if (log2base == 4 && len < BUFFER_SIZE) {
-      buf[len++] = 'x';
-    } else if (log2base == 1 && len < BUFFER_SIZE) {
-      buf[len++] = 'b';
+    if ((log2base == 4 || log2base == 1) && len < BUFFER_SIZE) {
+      buf[len++] = alphabet[17];  // x, X or b (for the corresponding conversion
+                                  // specifiers)
     }
     if (len < BUFFER_SIZE) {
       buf[len++] = '0';
@@ -122,7 +121,7 @@ int __fmt_ntoa2(int out(const char *, void *, size_t), void *arg,
     _npassert(count <= BUFFER_SIZE);
   }
   return __fmt_ntoa_format(out, arg, buf, len, neg, log2base, prec, width,
-                           flags);
+                           flags, alphabet);
 }
 
 int __fmt_ntoa(int out(const char *, void *, size_t), void *arg, va_list va,
diff --git a/test/libc/fmt/fmt_test.c b/test/libc/fmt/fmt_test.c
index bccd22e9a..d10e9c63e 100644
--- a/test/libc/fmt/fmt_test.c
+++ b/test/libc/fmt/fmt_test.c
@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/fmt/fmt.h"
+
 #include "libc/limits.h"
 #include "libc/log/log.h"
 #include "libc/math.h"
@@ -77,6 +78,26 @@ TEST(fmt, u) {
 TEST(fmt, x) {
   EXPECT_STREQ("0x01   ", _gc(xasprintf("%#-07.2x", 1)));
   EXPECT_STREQ("0x00136d  ", _gc(xasprintf("%#-010.6x", 4973)));
+  EXPECT_STREQ("0X1", _gc(xasprintf("%#X", 1)));
+  EXPECT_STREQ("0XA", _gc(xasprintf("%#X", 10)));
+  EXPECT_STREQ("0XFFFF", _gc(xasprintf("%#X", 65535)));
+  EXPECT_STREQ("0XABCDEF", _gc(xasprintf("%#X", 0xABCDEF)));
+  EXPECT_STREQ("0X1", _gc(xasprintf("%#hX", (short)1)));
+  EXPECT_STREQ("0XA", _gc(xasprintf("%#hX", (short)10)));
+  EXPECT_STREQ("0XFFFF", _gc(xasprintf("%#hX", (short)65535)));
+  EXPECT_STREQ("0XABCD", _gc(xasprintf("%#hX", (short)0xABCD)));
+  EXPECT_STREQ("          0X308C6705", _gc(xasprintf("%#20X", 814507781)));
+  EXPECT_STREQ("0X0000000000308C6705", _gc(xasprintf("%#020X", 814507781)));
+  EXPECT_STREQ("              0X6705",
+               _gc(xasprintf("%#20hX", (short)814507781)));
+  EXPECT_STREQ("0X000000000000006705",
+               _gc(xasprintf("%#020hX", (short)814507781)));
+  EXPECT_STREQ("            0XABCDEF", _gc(xasprintf("%#20X", 0xABCDEF)));
+  EXPECT_STREQ("0X000000000000ABCDEF", _gc(xasprintf("%#020X", 0xABCDEF)));
+  EXPECT_STREQ("              0XCDEF",
+               _gc(xasprintf("%#20hX", (short)0xABCDEF)));
+  EXPECT_STREQ("0X00000000000000CDEF",
+               _gc(xasprintf("%#020hX", (short)0xABCDEF)));
 }
 
 TEST(fmt, b) {
diff --git a/test/libc/fmt/palandprintf_test.c b/test/libc/fmt/palandprintf_test.c
index d37248bae..723082c74 100644
--- a/test/libc/fmt/palandprintf_test.c
+++ b/test/libc/fmt/palandprintf_test.c
@@ -310,8 +310,8 @@ TEST(sprintf, test_padding_pound_020) {
   EXPECT_STREQ("00000000037777777001", Format("%#020o", 4294966785U));
   EXPECT_STREQ("0x00000000001234abcd", Format("%#020x", 305441741));
   EXPECT_STREQ("0x0000000000edcb5433", Format("%#020x", 3989525555U));
-  EXPECT_STREQ("0x00000000001234ABCD", Format("%#020X", 305441741));
-  EXPECT_STREQ("0x0000000000EDCB5433", Format("%#020X", 3989525555U));
+  EXPECT_STREQ("0X00000000001234ABCD", Format("%#020X", 305441741));
+  EXPECT_STREQ("0X0000000000EDCB5433", Format("%#020X", 3989525555U));
 }
 
 TEST(sprintf, test_padding_pound_20) {
@@ -325,8 +325,8 @@ TEST(sprintf, test_padding_pound_20) {
   EXPECT_STREQ("        037777777001", Format("%#20o", 4294966785U));
   EXPECT_STREQ("          0x1234abcd", Format("%#20x", 305441741));
   EXPECT_STREQ("          0xedcb5433", Format("%#20x", 3989525555U));
-  EXPECT_STREQ("          0x1234ABCD", Format("%#20X", 305441741));
-  EXPECT_STREQ("          0xEDCB5433", Format("%#20X", 3989525555U));
+  EXPECT_STREQ("          0X1234ABCD", Format("%#20X", 305441741));
+  EXPECT_STREQ("          0XEDCB5433", Format("%#20X", 3989525555U));
 }
 
 TEST(sprintf, test_padding_20_point_5) {