diff --git a/.uberenv_config.json b/.uberenv_config.json
index 423cab102..f4fd2baf1 100644
--- a/.uberenv_config.json
+++ b/.uberenv_config.json
@@ -3,8 +3,8 @@
 "package_version" : "develop",
 "package_final_phase" : "initconfig",
 "package_source_dir" : "../..",
-"spack_url": "https://github.com/spack/spack.git",
-"spack_commit": "8ca06da26e0b57a54ecc3da13ff14b985297c6e0",
+"spack_url": "https://github.com/scheibelp/spack.git",
+"spack_commit": "a44b7841255a155c2828fe940a5b395292b3b957",
 "spack_packages_url": "https://github.com/spack/spack-packages.git",
 "spack_packages_commit": "98ae2e12518ff7605f0eea2901b6869981cfe160",
 "spack_configs_path": "scripts/spack/configs",
diff --git a/scripts/spack/configs/toss_4_x86_64_ib/spack.yaml b/scripts/spack/configs/toss_4_x86_64_ib/spack.yaml
index 15962a9da..ab594fc59 100644
--- a/scripts/spack/configs/toss_4_x86_64_ib/spack.yaml
+++ b/scripts/spack/configs/toss_4_x86_64_ib/spack.yaml
@@ -12,6 +12,9 @@ spack:
   # Regular TPLs do not need views
   view: false
 
+  concretizer:
+    compiler_mixing: false
+
   include:
   - ../defaults.yaml
   - ../versions.yaml