diff --git a/uri.c b/uri.c
index 31bbc7b6..57852f1b 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>
@@ -12,6 +11,10 @@
 #include "log.h"
 #include "xmalloc.h"
 
+enum {
+    HEX_DIGIT_INVALID = 16
+};
+
 static uint8_t
 nibble2hex(char c)
 {
@@ -27,8 +30,7 @@ nibble2hex(char c)
         return c - 'A' + 10;
     }
 
-    assert(false);
-    return 0;
+    return HEX_DIGIT_INVALID;
 }
 
 bool
@@ -199,7 +201,7 @@ uri_parse(const char *uri, size_t len,
             encoded_len -= prefix_len;
             decoded_len += prefix_len;
 
-            if (isxdigit(next[1]) && isxdigit(next[2])) {
+            if (nibble2hex(next[1]) <= 15 && nibble2hex(next[2]) <= 15) {
                 *p++ = nibble2hex(next[1]) << 4 | nibble2hex(next[2]);
                 decoded_len++;
                 encoded_len -= 3;