diff --git a/lib/spack/spack/compiler.py b/lib/spack/spack/compiler.py
index 72d56e590f4..d799ca01ea3 100644
--- a/lib/spack/spack/compiler.py
+++ b/lib/spack/spack/compiler.py
@@ -290,7 +290,7 @@ def __init__(
         operating_system,
         target,
         paths,
-        modules=None,
+        modules: Optional[List[str]] = None,
         alias=None,
         environment=None,
         extra_rpaths=None,
diff --git a/lib/spack/spack/schema/compilers.py b/lib/spack/spack/schema/compilers.py
index 1df696cda98..c894c2e32e3 100644
--- a/lib/spack/spack/schema/compilers.py
+++ b/lib/spack/spack/schema/compilers.py
@@ -52,7 +52,10 @@
                         "target": {"type": "string"},
                         "alias": {"anyOf": [{"type": "string"}, {"type": "null"}]},
                         "modules": {
-                            "anyOf": [{"type": "string"}, {"type": "null"}, {"type": "array"}]
+                            "anyOf": [
+                                {"type": "null"},
+                                {"type": "array", "items": {"type": "string"}},
+                            ]
                         },
                         "implicit_rpaths": {
                             "anyOf": [