diff --git a/pkgs/development/libraries/mimetic/default.nix b/pkgs/development/libraries/mimetic/default.nix
index 06b6653f4e76..0147a2973053 100644
--- a/pkgs/development/libraries/mimetic/default.nix
+++ b/pkgs/development/libraries/mimetic/default.nix
@@ -11,6 +11,8 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ cutee ];
 
+  patches = stdenv.lib.optional stdenv.isAarch64 ./narrowing.patch;
+
   meta = with stdenv.lib; {
     description = "MIME handling library";
     homepage    = "http://www.codesink.org/mimetic_mime_library.html";
diff --git a/pkgs/development/libraries/mimetic/narrowing.patch b/pkgs/development/libraries/mimetic/narrowing.patch
new file mode 100644
index 000000000000..676e29376c7d
--- /dev/null
+++ b/pkgs/development/libraries/mimetic/narrowing.patch
@@ -0,0 +1,32 @@
+diff -ru a/mimetic/codec/base64.cxx b/mimetic/codec/base64.cxx
+--- a/mimetic/codec/base64.cxx	2014-06-17 10:12:00.000000000 +0200
++++ b/mimetic/codec/base64.cxx	2020-07-30 20:54:10.212742011 +0200
+@@ -14,19 +14,19 @@
+     "0123456789+/=";
+ 
+ const char Base64::sDecTable[] = {
+-        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+-        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+-        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+-        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+-        -1,-1,-1,62,-1,-1,-1,63,52,53,
+-        54,55,56,57,58,59,60,61,-1,-1,
+-        -1, eq_sign, -1,-1,-1, 0, 1, 2, 3, 4, 
++        255,255,255,255,255,255,255,255,255,255,
++        255,255,255,255,255,255,255,255,255,255,
++        255,255,255,255,255,255,255,255,255,255,
++        255,255,255,255,255,255,255,255,255,255,
++        255,255,255,62,255,255,255,63,52,53,
++        54,55,56,57,58,59,60,61,255,255,
++        255, eq_sign, 255,255,255, 0, 1, 2, 3, 4, 
+          5, 6, 7, 8, 9,10,11,12,13,14,
+         15,16,17,18,19,20,21,22,23,24,
+-        25,-1,-1,-1,-1,-1,-1,26,27,28,
++        25,255,255,255,255,255,255,26,27,28,
+         29,30,31,32,33,34,35,36,37,38,
+         39,40,41,42,43,44,45,46,47,48,
+-        49,50,51,-1
++        49,50,51,255
+ };
+ 
+ const int Base64::sDecTableSz = sizeof(Base64::sDecTable) / sizeof(char);