diff --git a/uri.c b/uri.c
index 31bbc7b6..a0e3838a 100644
--- a/uri.c
+++ b/uri.c
@@ -2,7 +2,6 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <ctype.h>
 #include <limits.h>
 #include <unistd.h>
 #include <assert.h>
@@ -12,8 +11,12 @@
 #include "log.h"
 #include "xmalloc.h"
 
+enum {
+    HEX_DIGIT_INVALID = 16
+};
+
 static uint8_t
-nibble2hex(char c)
+hex2nibble(char c)
 {
     switch (c) {
     case '0': case '1': case '2': case '3': case '4':
@@ -27,8 +30,7 @@ nibble2hex(char c)
         return c - 'A' + 10;
     }
 
-    assert(false);
-    return 0;
+    return HEX_DIGIT_INVALID;
 }
 
 bool
@@ -199,8 +201,8 @@ uri_parse(const char *uri, size_t len,
             encoded_len -= prefix_len;
             decoded_len += prefix_len;
 
-            if (isxdigit(next[1]) && isxdigit(next[2])) {
-                *p++ = nibble2hex(next[1]) << 4 | nibble2hex(next[2]);
+            if (hex2nibble(next[1]) <= 15 && hex2nibble(next[2]) <= 15) {
+                *p++ = hex2nibble(next[1]) << 4 | hex2nibble(next[2]);
                 decoded_len++;
                 encoded_len -= 3;
                 encoded = next + 3;