diff --git a/pkgs/development/compilers/gcc/5/default.nix b/pkgs/development/compilers/gcc/5/default.nix
index e95ef92c3f65..c9780077835e 100644
--- a/pkgs/development/compilers/gcc/5/default.nix
+++ b/pkgs/development/compilers/gcc/5/default.nix
@@ -325,7 +325,13 @@ stdenv.mkDerivation ({
 
   NIX_LDFLAGS = stdenv.lib.optionalString  hostPlatform.isSunOS "-lm -ldl";
 
-  preConfigure = stdenv.lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''
+  preConfigure =
+    # Not sure why this is causing problems, now that the stdenv
+    # exports CPP=cpp the build fails with strange errors on darwin.
+    # https://github.com/NixOS/nixpkgs/issues/27889
+    stdenv.lib.optionalString stdenv.cc.isClang ''
+    unset CPP
+  '' + stdenv.lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''
     export NIX_LDFLAGS=`echo $NIX_LDFLAGS | sed -e s~$prefix/lib~$prefix/lib/amd64~g`
     export LDFLAGS_FOR_TARGET="-Wl,-rpath,$prefix/lib/amd64 $LDFLAGS_FOR_TARGET"
     export CXXFLAGS_FOR_TARGET="-Wl,-rpath,$prefix/lib/amd64 $CXXFLAGS_FOR_TARGET"