diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 8d39de389..b5f5d0cac 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -386,8 +386,10 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkOverlay = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
             try {
                 state->forceValue(v, pos);
-                if (!v.isLambda()
-                    || v.lambda.fun->hasFormals()
+                if (!v.isLambda()) {
+                    throw Error("overlay is not a function, but %s instead", showType(v));
+                }
+                if (v.lambda.fun->hasFormals()
                     || !argHasName(v.lambda.fun->arg, "final"))
                     throw Error("overlay does not take an argument named 'final'");
                 auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
diff --git a/tests/flakes/check.sh b/tests/flakes/check.sh
index 34b82c61c..0433e5335 100644
--- a/tests/flakes/check.sh
+++ b/tests/flakes/check.sh
@@ -25,6 +25,18 @@ EOF
 
 (! nix flake check $flakeDir)
 
+cat > $flakeDir/flake.nix <<EOF
+{
+  outputs = { self, ... }: {
+    overlays.x86_64-linux.foo = final: prev: {
+    };
+  };
+}
+EOF
+
+checkRes=$(nix flake check $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
+echo "$checkRes" | grepQuiet "error: overlay is not a function, but a set instead"
+
 cat > $flakeDir/flake.nix <<EOF
 {
   outputs = { self }: {