diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,8 @@
+* 0.12 [2018-01-28]
+
+URI encoding is now back to upper-case hexadecimal, as that is the preferred canonicalization, and the previous change caused issues with URI
+signing in at least amazonka.
+
 * 0.11 [2017-11-29]
 
 Remove dependency on blaze-builder. (Note that as a side effect of this, URI encoding is now using lower-case rather than uppercase hexadecimal.)
diff --git a/Network/HTTP/Types/URI.hs b/Network/HTTP/Types/URI.hs
--- a/Network/HTTP/Types/URI.hs
+++ b/Network/HTTP/Types/URI.hs
@@ -186,7 +186,11 @@
                     | ch >= 48 && ch <= 57  = True -- 0-9
       unreserved c = c `elem` extraUnreserved
 
-      h2 v = B.word8 37 `mappend` B.word8HexFixed v -- percent (%)
+      -- must be upper-case
+      h2 v = B.word8 37 `mappend` B.word8 (h a) `mappend` B.word8 (h b) -- 37 = %
+          where (a, b) = v `divMod` 16
+      h i | i < 10    = 48 + i -- zero (0)
+          | otherwise = 65 + i - 10 -- 65: A
 
 -- | Percent-encoding for URLs (using 'B.Builder').
 urlEncodeBuilder
diff --git a/http-types.cabal b/http-types.cabal
--- a/http-types.cabal
+++ b/http-types.cabal
@@ -1,5 +1,5 @@
 Name:                http-types
-Version:             0.11
+Version:             0.12
 Synopsis:            Generic HTTP types for Haskell (for both client and server code).
 Description:         Generic HTTP types for Haskell (for both client and server code).
 Homepage:            https://github.com/aristidb/http-types
@@ -16,7 +16,7 @@
 Source-repository this
   type: git
   location: https://github.com/aristidb/http-types.git
-  tag: 0.11
+  tag: 0.12
 
 Source-repository head
   type: git