diff --git a/pkgs/development/compilers/llvm/3.9/clang/default.nix b/pkgs/development/compilers/llvm/3.9/clang/default.nix
index e75da0e9717..6eadb91141d 100644
--- a/pkgs/development/compilers/llvm/3.9/clang/default.nix
+++ b/pkgs/development/compilers/llvm/3.9/clang/default.nix
@@ -16,7 +16,6 @@ let
     buildInputs = [ cmake libedit libxml2 llvm python ];
 
     cmakeFlags = [
-      "-DCMAKE_BUILD_TYPE=Release"
       "-DCMAKE_CXX_FLAGS=-std=c++11"
     ] ++
     # Maybe with compiler-rt this won't be needed?
diff --git a/pkgs/development/compilers/llvm/3.9/libc++/default.nix b/pkgs/development/compilers/llvm/3.9/libc++/default.nix
index 97cf65ad209..3b7caaa24bf 100644
--- a/pkgs/development/compilers/llvm/3.9/libc++/default.nix
+++ b/pkgs/development/compilers/llvm/3.9/libc++/default.nix
@@ -18,8 +18,7 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ cmake libcxxabi ] ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
-  cmakeFlags =
-    [ "-DCMAKE_BUILD_TYPE=Release"
+  cmakeFlags = [
       "-DLIBCXX_LIBCXXABI_LIB_PATH=${libcxxabi}/lib"
       "-DLIBCXX_LIBCPPABI_VERSION=2"
       "-DLIBCXX_CXX_ABI=libcxxabi"
diff --git a/pkgs/development/compilers/llvm/3.9/lldb.nix b/pkgs/development/compilers/llvm/3.9/lldb.nix
index bb92a6eed6c..0acef48f57b 100644
--- a/pkgs/development/compilers/llvm/3.9/lldb.nix
+++ b/pkgs/development/compilers/llvm/3.9/lldb.nix
@@ -41,7 +41,6 @@ stdenv.mkDerivation {
   hardeningDisable = [ "format" ];
 
   cmakeFlags = [
-    "-DCMAKE_BUILD_TYPE=Release"
     "-DLLVM_MAIN_INCLUDE_DIR=${llvm}/include"
   ];