diff --git a/lib/nghttp2_helper.c b/lib/nghttp2_helper.c
index 105c6bcb..87410a96 100644
--- a/lib/nghttp2_helper.c
+++ b/lib/nghttp2_helper.c
@@ -88,13 +88,78 @@ void* nghttp2_memdup(const void* src, size_t n)
   return dest;
 }
 
+static const int DOWNCASE_TBL[] = {
+  0 /* NUL  */, 1 /* SOH  */, 2 /* STX  */, 3 /* ETX  */,
+  4 /* EOT  */, 5 /* ENQ  */, 6 /* ACK  */, 7 /* BEL  */,
+  8 /* BS   */, 9 /* HT   */, 10 /* LF   */, 11 /* VT   */,
+  12 /* FF   */, 13 /* CR   */, 14 /* SO   */, 15 /* SI   */,
+  16 /* DLE  */, 17 /* DC1  */, 18 /* DC2  */, 19 /* DC3  */,
+  20 /* DC4  */, 21 /* NAK  */, 22 /* SYN  */, 23 /* ETB  */,
+  24 /* CAN  */, 25 /* EM   */, 26 /* SUB  */, 27 /* ESC  */,
+  28 /* FS   */, 29 /* GS   */, 30 /* RS   */, 31 /* US   */,
+  32 /* SPC  */, 33 /* !    */, 34 /* "    */, 35 /* #    */,
+  36 /* $    */, 37 /* %    */, 38 /* &    */, 39 /* '    */,
+  40 /* (    */, 41 /* )    */, 42 /* *    */, 43 /* +    */,
+  44 /* ,    */, 45 /* -    */, 46 /* .    */, 47 /* /    */,
+  48 /* 0    */, 49 /* 1    */, 50 /* 2    */, 51 /* 3    */,
+  52 /* 4    */, 53 /* 5    */, 54 /* 6    */, 55 /* 7    */,
+  56 /* 8    */, 57 /* 9    */, 58 /* :    */, 59 /* ;    */,
+  60 /* <    */, 61 /* =    */, 62 /* >    */, 63 /* ?    */,
+  64 /* @    */, 97 /* A    */, 98 /* B    */, 99 /* C    */,
+  100 /* D    */, 101 /* E    */, 102 /* F    */, 103 /* G    */,
+  104 /* H    */, 105 /* I    */, 106 /* J    */, 107 /* K    */,
+  108 /* L    */, 109 /* M    */, 110 /* N    */, 111 /* O    */,
+  112 /* P    */, 113 /* Q    */, 114 /* R    */, 115 /* S    */,
+  116 /* T    */, 117 /* U    */, 118 /* V    */, 119 /* W    */,
+  120 /* X    */, 121 /* Y    */, 122 /* Z    */, 91 /* [    */,
+  92 /* \    */, 93 /* ]    */, 94 /* ^    */, 95 /* _    */,
+  96 /* `    */, 97 /* a    */, 98 /* b    */, 99 /* c    */,
+  100 /* d    */, 101 /* e    */, 102 /* f    */, 103 /* g    */,
+  104 /* h    */, 105 /* i    */, 106 /* j    */, 107 /* k    */,
+  108 /* l    */, 109 /* m    */, 110 /* n    */, 111 /* o    */,
+  112 /* p    */, 113 /* q    */, 114 /* r    */, 115 /* s    */,
+  116 /* t    */, 117 /* u    */, 118 /* v    */, 119 /* w    */,
+  120 /* x    */, 121 /* y    */, 122 /* z    */, 123 /* {    */,
+  124 /* |    */, 125 /* }    */, 126 /* ~    */, 127 /* DEL  */,
+  128 /* 0x80 */, 129 /* 0x81 */, 130 /* 0x82 */, 131 /* 0x83 */,
+  132 /* 0x84 */, 133 /* 0x85 */, 134 /* 0x86 */, 135 /* 0x87 */,
+  136 /* 0x88 */, 137 /* 0x89 */, 138 /* 0x8a */, 139 /* 0x8b */,
+  140 /* 0x8c */, 141 /* 0x8d */, 142 /* 0x8e */, 143 /* 0x8f */,
+  144 /* 0x90 */, 145 /* 0x91 */, 146 /* 0x92 */, 147 /* 0x93 */,
+  148 /* 0x94 */, 149 /* 0x95 */, 150 /* 0x96 */, 151 /* 0x97 */,
+  152 /* 0x98 */, 153 /* 0x99 */, 154 /* 0x9a */, 155 /* 0x9b */,
+  156 /* 0x9c */, 157 /* 0x9d */, 158 /* 0x9e */, 159 /* 0x9f */,
+  160 /* 0xa0 */, 161 /* 0xa1 */, 162 /* 0xa2 */, 163 /* 0xa3 */,
+  164 /* 0xa4 */, 165 /* 0xa5 */, 166 /* 0xa6 */, 167 /* 0xa7 */,
+  168 /* 0xa8 */, 169 /* 0xa9 */, 170 /* 0xaa */, 171 /* 0xab */,
+  172 /* 0xac */, 173 /* 0xad */, 174 /* 0xae */, 175 /* 0xaf */,
+  176 /* 0xb0 */, 177 /* 0xb1 */, 178 /* 0xb2 */, 179 /* 0xb3 */,
+  180 /* 0xb4 */, 181 /* 0xb5 */, 182 /* 0xb6 */, 183 /* 0xb7 */,
+  184 /* 0xb8 */, 185 /* 0xb9 */, 186 /* 0xba */, 187 /* 0xbb */,
+  188 /* 0xbc */, 189 /* 0xbd */, 190 /* 0xbe */, 191 /* 0xbf */,
+  192 /* 0xc0 */, 193 /* 0xc1 */, 194 /* 0xc2 */, 195 /* 0xc3 */,
+  196 /* 0xc4 */, 197 /* 0xc5 */, 198 /* 0xc6 */, 199 /* 0xc7 */,
+  200 /* 0xc8 */, 201 /* 0xc9 */, 202 /* 0xca */, 203 /* 0xcb */,
+  204 /* 0xcc */, 205 /* 0xcd */, 206 /* 0xce */, 207 /* 0xcf */,
+  208 /* 0xd0 */, 209 /* 0xd1 */, 210 /* 0xd2 */, 211 /* 0xd3 */,
+  212 /* 0xd4 */, 213 /* 0xd5 */, 214 /* 0xd6 */, 215 /* 0xd7 */,
+  216 /* 0xd8 */, 217 /* 0xd9 */, 218 /* 0xda */, 219 /* 0xdb */,
+  220 /* 0xdc */, 221 /* 0xdd */, 222 /* 0xde */, 223 /* 0xdf */,
+  224 /* 0xe0 */, 225 /* 0xe1 */, 226 /* 0xe2 */, 227 /* 0xe3 */,
+  228 /* 0xe4 */, 229 /* 0xe5 */, 230 /* 0xe6 */, 231 /* 0xe7 */,
+  232 /* 0xe8 */, 233 /* 0xe9 */, 234 /* 0xea */, 235 /* 0xeb */,
+  236 /* 0xec */, 237 /* 0xed */, 238 /* 0xee */, 239 /* 0xef */,
+  240 /* 0xf0 */, 241 /* 0xf1 */, 242 /* 0xf2 */, 243 /* 0xf3 */,
+  244 /* 0xf4 */, 245 /* 0xf5 */, 246 /* 0xf6 */, 247 /* 0xf7 */,
+  248 /* 0xf8 */, 249 /* 0xf9 */, 250 /* 0xfa */, 251 /* 0xfb */,
+  252 /* 0xfc */, 253 /* 0xfd */, 254 /* 0xfe */, 255 /* 0xff */,
+};
+
 void nghttp2_downcase(uint8_t *s, size_t len)
 {
   size_t i;
   for(i = 0; i < len; ++i) {
-    if('A' <= s[i] && s[i] <= 'Z') {
-      s[i] += 'a'-'A';
-    }
+    s[i] = DOWNCASE_TBL[s[i]];
   }
 }