Fixes #3467

src/TemplateMap.js

@@ -585,7 +585,10 @@ class TemplateMap {
 		} else {
 			result = this.collection.getFilteredByTag(tag);
 		}
-		debug(`Collection: collections.${tag || "all"} size: ${result.length}`);
+
+		// May not return an array (can be anything)
+		// https://www.11ty.dev/docs/collections-api/#return-values
+		debug(`Collection: collections.${tag || "all"} size: ${result?.length}`);
 
 		return result;
 	}
@@ -606,7 +609,9 @@ class TemplateMap {
 		// This works with async now
 		let result = await configCollections[name](this.collection);
 
-		debug(`Collection: collections.${name} size: ${result.length}`);
+		// May not return an array (can be anything)
+		// https://www.11ty.dev/docs/collections-api/#return-values
+		debug(`Collection: collections.${name} size: ${result?.length}`);
 		return result;
 	}
 

test/Issue3467Test.js

@@ -0,0 +1,56 @@
+import test from "ava";
+import Eleventy from "../src/Eleventy.js";
+
+test("Empty collections api #3467 (return undefined)", async (t) => {
+	let elev = new Eleventy("./test/stubs-virtual", "./test/stubs-virtual/_site", {
+		config: function (eleventyConfig) {
+			eleventyConfig.addTemplate("virtual.md", `# Hello`);
+
+      eleventyConfig.addCollection("brokenCollection", function(collection) {
+        // returns nothing
+      });
+		},
+	});
+
+	let results = await elev.toJSON();
+
+	t.deepEqual(results.length, 1);
+	t.deepEqual(results[0].content.trim(), `<h1>Hello</h1>`);
+	t.deepEqual(results[0].rawInput, `# Hello`);
+});
+
+test("Empty collections api #3467 (return false)", async (t) => {
+	let elev = new Eleventy("./test/stubs-virtual", "./test/stubs-virtual/_site", {
+		config: function (eleventyConfig) {
+			eleventyConfig.addTemplate("virtual.md", `# Hello`);
+
+      eleventyConfig.addCollection("brokenCollection", function(collection) {
+        return false;
+      });
+		},
+	});
+
+	let results = await elev.toJSON();
+
+	t.deepEqual(results.length, 1);
+	t.deepEqual(results[0].content.trim(), `<h1>Hello</h1>`);
+	t.deepEqual(results[0].rawInput, `# Hello`);
+});
+
+test("Empty collections api #3467 (return empty string)", async (t) => {
+	let elev = new Eleventy("./test/stubs-virtual", "./test/stubs-virtual/_site", {
+		config: function (eleventyConfig) {
+			eleventyConfig.addTemplate("virtual.md", `# Hello`);
+
+      eleventyConfig.addCollection("brokenCollection", function(collection) {
+        return "";
+      });
+		},
+	});
+
+	let results = await elev.toJSON();
+
+	t.deepEqual(results.length, 1);
+	t.deepEqual(results[0].content.trim(), `<h1>Hello</h1>`);
+	t.deepEqual(results[0].rawInput, `# Hello`);
+});