diff --git a/pkgs/development/compilers/gcc-4.5/default.nix b/pkgs/development/compilers/gcc-4.5/default.nix
index b3bd431fbc95..5f99ad89434c 100644
--- a/pkgs/development/compilers/gcc-4.5/default.nix
+++ b/pkgs/development/compilers/gcc-4.5/default.nix
@@ -147,13 +147,15 @@ stdenv.mkDerivation ({
 
   postPatch =
     if (stdenv.system == "i586-pc-gnu"
+        || (libcCross != null)                 # e.g., building `gcc.hostDrv'
         || (cross != null && cross.config == "i586-pc-gnu"
             && libcCross != null))
     then
       # On GNU/Hurd glibc refers to Hurd & Mach headers and libpthread is not
       # in glibc, so add the right `-I' flags to the default spec string.
+      assert libcCross != null -> libpthreadCross != null;
       let
-        libc = if cross != null then libcCross else stdenv.glibc;
+        libc = if libcCross != null then libcCross else stdenv.glibc;
         gnu_h = "gcc/config/gnu.h";
         i386_gnu_h = "gcc/config/i386/gnu.h";
         extraCPPDeps =
@@ -186,7 +188,7 @@ stdenv.mkDerivation ({
       # On NixOS, use the right path to the dynamic linker instead of
       # `/lib/ld*.so'.
       let
-        libc = if (cross != null && libcCross != null) then libcCross else stdenv.gcc.libc;
+        libc = if (libcCross != null) then libcCross else stdenv.gcc.libc;
       in
         '' echo "fixing the \`GLIBC_DYNAMIC_LINKER' and \`UCLIBC_DYNAMIC_LINKER' macros..."
            for header in "gcc/config/"*-gnu.h "gcc/config/"*"/"*.h
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index a6aeee713df0..1b3f1b4b6721 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -1847,6 +1847,16 @@ let
     # bootstrapping a profiled compiler does not work in the sheevaplug:
     # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43944
     profiledCompiler = if stdenv.system == "armv5tel-linux" then false else true;
+
+    # When building `gcc.hostDrv' (a "Canadian cross", with host == target
+    # and host != build), `cross' must be null but the cross-libc must still
+    # be passed.
+    cross = null;
+    libcCross = if crossSystem != null then libcCross else null;
+    libpthreadCross =
+      if crossSystem != null && crossSystem.config == "i586-pc-gnu"
+      then gnu.libpthreadCross
+      else null;
   }));
 
   gcc46_real = lowPrio (wrapGCC (callPackage ../development/compilers/gcc-4.6 {