diff --git a/index.js b/index.js
index eb1571d..8047347 100644
--- a/index.js
+++ b/index.js
@@ -1,5 +1,7 @@
 "use strict";
 
+const { validateArguments, required, oneOf, arrayOf, allowExtraProperties, ValidationError } = require("validatem");
+
 const createTransform = require("./src/transform");
 const createCssDepsStream = require("./src/phase-streams/deps-css");
 const createSyntaxHideStream = require("./src/phase-streams/syntax-hide");
@@ -7,6 +9,12 @@ const createSyntaxUnhideStream = require("./src/phase-streams/syntax-unhide");
 const createKahnSortingStream = require("./src/phase-streams/sort-kahn");
 const createDedupeBundleCssStream = require("./src/phase-streams/dedupe-bundle-css");
 
+function isPostcssPlugin(value) {
+	if (value.postcssPlugin == null || value.postcssVersion == null || typeof value !== "function") {
+		throw new ValidationError(`Must be a PostCSS plugin`);
+	}
+}
+
 // FIXME: When there's >1 icssify instance set up on the Browserify instance (can this happen when a module specifies its own dependency on icssify?), either throw an error (if incompatible?) or merge the two somehow, so that there's only one resulting .css file
 
 function setupPipeline(browserify, options) {
@@ -35,6 +43,17 @@ function setupPipeline(browserify, options) {
 }
 
 module.exports = function (browserify, options) {
+	console.log(options);
+	
+	validateArguments(arguments, [
+		[ "browserify" ],
+		[ "options", allowExtraProperties({
+			mode: oneOf([ "local", "global" ]),
+			before: arrayOf([ required, isPostcssPlugin ]),
+			after: arrayOf([ required, isPostcssPlugin ])
+		})]
+	]);
+
 	browserify.transform(createTransform, options);
 
 	browserify.on("reset", () => {