diff --git a/pkgs/development/libraries/boehm-gc/7.6.6.nix b/pkgs/development/libraries/boehm-gc/7.6.6.nix
index da71e40187f4..68f5d7afcf49 100644
--- a/pkgs/development/libraries/boehm-gc/7.6.6.nix
+++ b/pkgs/development/libraries/boehm-gc/7.6.6.nix
@@ -21,7 +21,7 @@ stdenv.mkDerivation rec {
   separateDebugInfo = stdenv.isLinux;
 
   preConfigure = stdenv.lib.optionalString (stdenv.hostPlatform.libc == "musl") ''
-    export NIX_CFLAGS_COMPILE+="-D_GNU_SOURCE -DUSE_MMAP -DHAVE_DL_ITERATE_PHDR"
+    export NIX_CFLAGS_COMPILE+=" -D_GNU_SOURCE -DUSE_MMAP -DHAVE_DL_ITERATE_PHDR"
   '';
 
   patches = [ (fetchpatch {