diff --git a/.changeset/validate-external-composes.md b/.changeset/validate-external-composes.md
new file mode 100644
index 000000000..19842fa6c
--- /dev/null
+++ b/.changeset/validate-external-composes.md
@@ -0,0 +1,5 @@
+---
+"@modular-css/processor": patch
+---
+
+Fail with `Invalid composes reference` when `composes: x from "file.css"` references a class that doesn't exist in that file, instead of crashing later with `composition is not iterable`.
diff --git a/packages/processor/plugins/composition.js b/packages/processor/plugins/composition.js
index 1d434fc2c..d86090f7a 100644
--- a/packages/processor/plugins/composition.js
+++ b/packages/processor/plugins/composition.js
@@ -75,6 +75,16 @@ module.exports = () => ({
                         if(source) {
                             // External refs should already exist, so they don't get added
                             ref = selectorKey(source, name);
+
+                            if(!files[source].classes[name]) {
+                                const rel = relative(processor.options.cwd, source);
+
+                                throw decl.error(
+                                    `Invalid composes reference, .${name} does not exist in ${rel}`, {
+                                    word: name,
+                                }
+                                );
+                            }
                         } else if(global) {
                             ref = processor._addGlobal(name);
                         } else {
diff --git a/packages/processor/test/composition.test.js b/packages/processor/test/composition.test.js
index 52488c6d6..a52244788 100644
--- a/packages/processor/test/composition.test.js
+++ b/packages/processor/test/composition.test.js
@@ -37,6 +37,16 @@ describe("/processor.js", () => {
             );
         });
 
+        it("should fail if a composition references a non-existant class in another file", async (t) => {
+            await t.assert.rejects(
+                async () => processor.string(
+                    path.join(__dirname, "specimens/invalid-external-composition.css"),
+                    `.a { composes: nope from "./simple.css"; }`
+                ),
+                /Invalid composes reference, \.nope does not exist in packages\/processor\/test\/specimens\/simple\.css/
+            );
+        });
+
         it("should fail if a composition references a non-existant file", async (t) => {
             await t.assert.rejects(
                 async () => processor.string(