diff --git a/pkgs/by-name/ac/ac-library/fix-char-signedness-tests.patch b/pkgs/by-name/ac/ac-library/fix-char-signedness-tests.patch
new file mode 100644
index 000000000000..7373784dbe43
--- /dev/null
+++ b/pkgs/by-name/ac/ac-library/fix-char-signedness-tests.patch
@@ -0,0 +1,24 @@
+diff --git i/test/unittest/type_traits_test.cpp w/test/unittest/type_traits_test.cpp
+index 164cc79..123b3a2 100644
+--- i/test/unittest/type_traits_test.cpp
++++ w/test/unittest/type_traits_test.cpp
+@@ -1,5 +1,6 @@
+ #include "atcoder/internal_type_traits"
+ 
++#include <climits>
+ #include <type_traits>
+ 
+ #include <gtest/gtest.h>
+@@ -44,7 +45,12 @@ static_assert(internal::is_unsigned_int<unsigned long long>::value, "");
+ static_assert(!internal::is_signed_int<A>::value, "");
+ static_assert(!internal::is_unsigned_int<A>::value, "");
+ 
++#if CHAR_MIN < 0
+ static_assert(is_same<unsigned char, internal::to_unsigned_t<char>>::value, "");
++#else
++static_assert(is_same<char, internal::to_unsigned_t<char>>::value, "");
++#endif
++
+ static_assert(
+     is_same<unsigned char, internal::to_unsigned_t<signed char>>::value,
+     "");
diff --git a/pkgs/by-name/ac/ac-library/package.nix b/pkgs/by-name/ac/ac-library/package.nix
index b112b8128a81..bdbb3e5d2355 100644
--- a/pkgs/by-name/ac/ac-library/package.nix
+++ b/pkgs/by-name/ac/ac-library/package.nix
@@ -18,7 +18,12 @@ stdenv.mkDerivation (finalAttrs: {
     fetchSubmodules = true;
     hash = "sha256-1wwzN/JPS6daj1vDFuEN5z20tMdLfMvEKti0sxCVlHA=";
   };
-
+  patches = [
+    # Fix type_traits_test assumptions about char signedness on platforms
+    # where char is unsigned by default (e.g. aarch64-linux).
+    # Reported upstream: https://github.com/atcoder/ac-library/issues/191
+    ./fix-char-signedness-tests.patch
+  ];
   outputs = [
     "dev"
     "out"
@@ -60,7 +65,6 @@ stdenv.mkDerivation (finalAttrs: {
   env = {
     NIX_CFLAGS_COMPILE = toString [
       "-Wno-error=array-bounds"
-      "-Wno-character-conversion"
     ];
   };