diff --git a/pkgs/development/libraries/x265/default.nix b/pkgs/development/libraries/x265/default.nix
index b4a7777b8911..7e81d8ff6115 100644
--- a/pkgs/development/libraries/x265/default.nix
+++ b/pkgs/development/libraries/x265/default.nix
@@ -34,6 +34,8 @@ let
     (mkFlag ppaSupport "ENABLE_PPA")
     (mkFlag vtuneSupport "ENABLE_VTUNE")
     (mkFlag werrorSupport "WARNINGS_AS_ERRORS")
+    # Potentially riscv cross could be fixed by providing the correct CMAKE_SYSTEM_PROCESSOR flag
+    (mkFlag (!(isCross && stdenv.hostPlatform.isRiscV)) "ENABLE_ASSEMBLY")
   ];
 
   cmakeStaticLibFlags = [