diff --git a/lib/src/modules/default.test.nix b/lib/src/modules/default.test.nix
index d05d4b7..9b816f2 100644
--- a/lib/src/modules/default.test.nix
+++ b/lib/src/modules/default.test.nix
@@ -200,6 +200,57 @@ in
         in
         !value;
     };
+
+    "cannot mix include and module keys" =
+      let
+        # Simple backtracking algorithm to generate all combinations of a list's
+        # elements without ordering, in .
+        combinations = set:
+          let
+            n = builtins.length set;
+            genIndices = combinations':
+              let
+                prev = builtins.head combinations';
+                k = builtins.length prev;
+                first = builtins.head prev;
+                next' =
+                  # Generate the next combination of length k
+                  if k > 0 && first+1 < n then [(first+1)] ++ (builtins.tail prev)
+                  # We have generated all combinations of length k, generate the first
+                  # (trivial) combination of length k+1
+                  else builtins.genList (i: i) (k+1);
+                next = builtins.deepSeq next' next';
+              in
+              # Generate the very first (trivial) combination
+              if combinations' == [ ] then genIndices [ [ ] ]
+              # We have generated all combinations of length 0 to n, return
+              else if k >= n then combinations'
+              # Generate the next combination and continue
+              else genIndices ([next] ++ combinations');
+            select = indices: builtins.map (builtins.elemAt set) indices;
+          in
+            if set == [ ] then [ ]
+            else builtins.map select (genIndices []);
+
+        keys = combinations (lib.modules.VALID_KEYS ++ lib.modules.VALID_INCLUDE_KEYS);
+        sets =
+          builtins.map
+          (names:
+            builtins.listToAttrs
+            (builtins.map (name: { value = null; inherit name; }) names)
+          )
+          keys;
+        validated =
+          builtins.map
+          (set: {
+            inherit set;
+            include = lib.modules.validate.include set;
+            module = lib.modules.validate.keys set;
+          })
+          sets;
+        bothValid = {include, module, ...}: include && module;
+      in
+      ! builtins.any bothValid validated;
   };
 
   "normalize" = {