diff --git a/mwparserfromhell/parser/ctokenizer/tok_parse.c b/mwparserfromhell/parser/ctokenizer/tok_parse.c
index 879a5db..d36ce56 100644
--- a/mwparserfromhell/parser/ctokenizer/tok_parse.c
+++ b/mwparserfromhell/parser/ctokenizer/tok_parse.c
@@ -30,7 +30,7 @@ SOFTWARE.
 #define DIGITS    "0123456789"
 #define HEXDIGITS "0123456789abcdefABCDEF"
 #define ALPHANUM  "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
-#define URISCHEME "abcdefghijklmnopqrstuvwxyz0123456789+.-"
+#define URISCHEME "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+.-"
 
 #define MAX_BRACES 255
 #define MAX_ENTITY_SIZE 8
diff --git a/mwparserfromhell/parser/tokenizer.py b/mwparserfromhell/parser/tokenizer.py
index 6acfb8d..c48e180 100644
--- a/mwparserfromhell/parser/tokenizer.py
+++ b/mwparserfromhell/parser/tokenizer.py
@@ -366,7 +366,7 @@ class Tokenizer:
             self._emit_text("//")
             self._head += 2
         else:
-            valid = "abcdefghijklmnopqrstuvwxyz0123456789+.-"
+            valid = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+.-"
             all_valid = lambda: all(char in valid for char in self._read())
             scheme = ""
             while self._read() is not self.END and all_valid():
@@ -386,7 +386,7 @@ class Tokenizer:
 
     def _parse_free_uri_scheme(self):
         """Parse the URI scheme of a free (no brackets) external link."""
-        valid = "abcdefghijklmnopqrstuvwxyz0123456789+.-"
+        valid = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+.-"
         scheme = []
         try:
             # We have to backtrack through the textbuffer looking for our
diff --git a/tests/tokenizer/external_links.mwtest b/tests/tokenizer/external_links.mwtest
index 7fed7b6..ca64fd0 100644
--- a/tests/tokenizer/external_links.mwtest
+++ b/tests/tokenizer/external_links.mwtest
@@ -562,3 +562,17 @@ name:   brackets_terminated_by_right_angle
 label:  an external link terminated by a right angle
 input:  "[http://foo>bar]"
 output: [ExternalLinkOpen(brackets=True), Text(text="http://foo"), ExternalLinkSeparator(suppress_space=True), Text(text=">bar"), ExternalLinkClose()]
+
+---
+
+name:   scheme_case
+label:  a free link with uppercase letters in the URL scheme
+input:  "HtTp://example.com/"
+output: [ExternalLinkOpen(brackets=False), Text(text="HtTp://example.com/"), ExternalLinkClose()]
+
+---
+
+name:   bracket_scheme_case
+label:  an external link with uppercase letters in the URL scheme
+input:  "[HtTp://example.com/]"
+output: [ExternalLinkOpen(brackets=True), Text(text="HtTp://example.com/"), ExternalLinkClose()]