diff --git a/src/TemplateMap.js b/src/TemplateMap.js
index e71bc7f46..5ded43f1f 100644
--- a/src/TemplateMap.js
+++ b/src/TemplateMap.js
@@ -409,18 +409,6 @@ class TemplateMap {
 				} else {
 					let counter = 0;
 					for (let page of map._pages) {
-						// Data Schema callback #879
-						if (typeof page.data[this.config.keys.dataSchema] === "function") {
-							try {
-								await page.data[this.config.keys.dataSchema](page.data);
-							} catch (e) {
-								throw new EleventyDataSchemaError(
-									`Error in the data schema for: ${map.inputPath} (via \`eleventyDataSchema\`)`,
-									e,
-								);
-							}
-						}
-
 						// Copy outputPath to map entry
 						if (!map.outputPath) {
 							map.outputPath = page.outputPath;
@@ -478,6 +466,7 @@ class TemplateMap {
 			};
 		});
 
+		await this.runDataSchemas(orderedMap);
 		await this.populateContentDataInMap(orderedMap);
 
 		this.populateCollectionsWithContent();
@@ -534,6 +523,28 @@ class TemplateMap {
 		return orderedMap;
 	}
 
+	async runDataSchemas(orderedMap) {
+		for (let map of orderedMap) {
+			if (!map._pages) {
+				continue;
+			}
+
+			for (let pageEntry of map._pages) {
+				// Data Schema callback #879
+				if (typeof pageEntry.data[this.config.keys.dataSchema] === "function") {
+					try {
+						await pageEntry.data[this.config.keys.dataSchema](pageEntry.data);
+					} catch (e) {
+						throw new EleventyDataSchemaError(
+							`Error in the data schema for: ${map.inputPath} (via \`eleventyDataSchema\`)`,
+							e,
+						);
+					}
+				}
+			}
+		}
+	}
+
 	async populateContentDataInMap(orderedMap) {
 		let usedTemplateContentTooEarlyMap = [];
 		for (let map of orderedMap) {
diff --git a/test/EleventyTest.js b/test/EleventyTest.js
index bddda9c3e..fb37d126b 100644
--- a/test/EleventyTest.js
+++ b/test/EleventyTest.js
@@ -1201,3 +1201,24 @@ test("Eleventy data schema (fails, using zod) #879", async (t) => {
 
   t.is(e.originalError.toString(), 'Validation error: Expected boolean, received number at "draft", or Expected undefined, received number at "draft"');
 });
+
+test("Eleventy data schema has access to custom collections created via API #879", async (t) => {
+  t.plan(2);
+  let elev = new Eleventy("./test/stubs-virtual/", undefined, {
+    config: eleventyConfig => {
+      eleventyConfig.addCollection("userCollection", function (collection) {
+        return collection.getAll();
+      });
+
+      eleventyConfig.addTemplate("index1.html", "", {
+        eleventyDataSchema: function(data) {
+          t.is(data.collections.userCollection.length, 1);
+        }
+      });
+    }
+  });
+  elev.disableLogger();
+
+  let results = await elev.toJSON();
+  t.is(results.length, 1);
+});