diff --git a/pkgs/development/compilers/llvm/6/llvm.nix b/pkgs/development/compilers/llvm/6/llvm.nix
index 4f7a2835cc93..256b9120c826 100644
--- a/pkgs/development/compilers/llvm/6/llvm.nix
+++ b/pkgs/development/compilers/llvm/6/llvm.nix
@@ -16,6 +16,7 @@
 , debugVersion ? false
 , enableManpages ? false
 , enableSharedLibraries ? true
+, enableWasm ? true
 , darwin
 }:
 
@@ -115,7 +116,9 @@ in stdenv.mkDerivation (rec {
 
     "-DCOMPILER_RT_BUILD_SANITIZERS=OFF"
     "-DCOMPILER_RT_BUILD_XRAY=OFF"
-  ];
+  ] ++ stdenv.lib.optional enableWasm
+   "-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly"
+  ;
 
   postBuild = ''
     rm -fR $out