diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 2d5b66487ebf3f8aaca3ae3c2bffff081d53417f..dcea1478141eb328a538b188e0d5c3a9c4af6c2c 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -390,8 +390,10 @@ def _proper_compiler_style(cspec, arch):
                 c for c in matches
                 if _proper_compiler_style(c, spec.architecture)).copy()
         except StopIteration:
-            raise UnavailableCompilerVersionError(spec.compiler,
-                                                  arch.platform_os)
+            raise UnavailableCompilerVersionError(
+                spec.compiler, spec.architecture.platform_os
+            )
+
         assert(spec.compiler.concrete)
         return True  # things changed.