diff --git a/templatify.ts b/templatify.ts
index 877c2ca..cf62262 100644
--- a/templatify.ts
+++ b/templatify.ts
@@ -1,71 +1,100 @@
-import * as yaml from 'js-yaml';
-import * as Handlebars from 'handlebars';
-import * as fs from 'fs-extra';
-import * as path from 'path';
-
-const config = {
-  templatedFiles: [
-    'activity-subgraph/subgraph.yaml',
-    'pnl-subgraph/subgraph.yaml',
-    'oi-subgraph/subgraph.yaml',
-    'fpmm-subgraph/subgraph.yaml',
-    'orderbook-subgraph/subgraph.yaml',
-    'sports-oracle-subgraph/subgraph.yaml',
-    'wallet-subgraph/subgraph.yaml',
-    'fee-module-subgraph/subgraph.yaml',
-    'common/constants.ts',
-  ],
-};
-
-Handlebars.registerHelper('lowercase', function (str) {
-  if (str && typeof str == 'string') {
-    return str.toLowerCase();
-  }
-  return '';
-});
-
-// function getNetworkNameForSubgraph(): string | null {
-//   switch (process.env.SUBGRAPH) {
-//     case 'tomafrench/polymarket':
-//       return 'mainnet';
-//     case 'Polymarket/polymarket':
-//       return 'mainnet';
-//     case 'Polymarket/polymarket-matic':
-//       return 'matic';
-//     case 'Polymarket/polymarket-mumbai':
-//       return 'mumbai';
-//     default:
-//       return null;
-//   }
-// }
-
-(async (): Promise<void> => {
-  console.log('Starting...');
-  const networksFilePath = path.join(__dirname, 'networks.yaml');
-  const networks: any = yaml.load(
-    await fs.readFile(networksFilePath, { encoding: 'utf-8' }),
-  );
-
-  const networkName = process.argv[2];
-  console.log(`Network: ${networkName}`);
-  const network = { ...networks[networkName || ''], networkName };
-
-  if (!networkName) {
-    throw new Error(
-      'Please set either a "NETWORK_NAME" or a "SUBGRAPH" environment variable',
-    );
-  }
-
-  // eslint-disable-next-line no-restricted-syntax
-  for (const templatedFile of config.templatedFiles) {
-    console.log(templatedFile);
-    const templatedFileDesc = templatedFile.split('.');
-    const template = fs
-      .readFileSync(`${templatedFileDesc[0]}.template.${templatedFileDesc[1]}`)
-      .toString();
-    const result = Handlebars.compile(template, {})(network);
-    fs.writeFileSync(`${templatedFileDesc[0]}.${templatedFileDesc[1]}`, result);
-  }
-
-  console.log(`🎉 subgraph successfully generated for ${networkName}\n`);
-})();
+import * as yaml from 'js-yaml';
+import * as Handlebars from 'handlebars';
+import * as fs from 'fs-extra';
+import * as path from 'path';
+
+const config = {
+  templatedFiles: [
+    'activity-subgraph/subgraph.yaml',
+    'pnl-subgraph/subgraph.yaml',
+    'oi-subgraph/subgraph.yaml',
+    'fpmm-subgraph/subgraph.yaml',
+    'orderbook-subgraph/subgraph.yaml',
+    'sports-oracle-subgraph/subgraph.yaml',
+    'wallet-subgraph/subgraph.yaml',
+    'fee-module-subgraph/subgraph.yaml',
+    'common/constants.ts',
+  ],
+};
+
+Handlebars.registerHelper('lowercase', function (str) {
+  if (str && typeof str == 'string') {
+    return str.toLowerCase();
+  }
+  return '';
+});
+
+// function getNetworkNameForSubgraph(): string | null {
+//   switch (process.env.SUBGRAPH) {
+//     case 'tomafrench/polymarket':
+//       return 'mainnet';
+//     case 'Polymarket/polymarket':
+//       return 'mainnet';
+//     case 'Polymarket/polymarket-matic':
+//       return 'matic';
+//     case 'Polymarket/polymarket-mumbai':
+//       return 'mumbai';
+//     default:
+//       return null;
+//   }
+// }
+
+(async (): Promise<void> => {
+  console.log('Starting...');
+  const networksFilePath = path.join(__dirname, 'networks.yaml');
+  const networks: any = yaml.load(
+    await fs.readFile(networksFilePath, { encoding: 'utf-8' }),
+  );
+
+  // Accept the network as either the first positional argument or the
+  // NETWORK_NAME environment variable.
+  //
+  // Only `process.argv[2]` was read before, while the error message told the
+  // user to set NETWORK_NAME. That mismatch was not cosmetic: the
+  // `prepare:mainnet`, `prepare:matic` and `prepare:mumbai` package scripts pass
+  // the network via `NETWORK_NAME=...` and no argument at all, so all three
+  // always threw before generating anything. Only `templatify:matic`, which
+  // passes the network positionally, worked.
+  const networkName = process.argv[2] || process.env.NETWORK_NAME;
+
+  // Validation happens before `networks[networkName]` is read. The previous
+  // order dereferenced the lookup on the line above the guard, so it ran with
+  // `networks['']`, and the `SUBGRAPH` half of the message referred to a
+  // variable this script has never read (see the commented-out
+  // getNetworkNameForSubgraph above, which is where that came from).
+  if (!networkName) {
+    throw new Error(
+      'Please pass the network as the first argument (e.g. `ts-node ./templatify.ts matic`) ' +
+        'or set the "NETWORK_NAME" environment variable.',
+    );
+  }
+
+  // An unknown key used to yield `{...undefined}`, i.e. an empty context, and
+  // Handlebars renders a missing value as the empty string. That produced a
+  // syntactically valid subgraph.yaml with blank contract addresses and
+  // `startBlock:` unset -- a manifest that deploys and then indexes nothing,
+  // which is far more expensive to diagnose than a failed generate.
+  if (!networks[networkName]) {
+    throw new Error(
+      `Unknown network "${networkName}". networks.yaml defines: ${Object.keys(
+        networks,
+      ).join(', ')}.`,
+    );
+  }
+
+  console.log(`Network: ${networkName}`);
+  const network = { ...networks[networkName], networkName };
+
+  // eslint-disable-next-line no-restricted-syntax
+  for (const templatedFile of config.templatedFiles) {
+    console.log(templatedFile);
+    const templatedFileDesc = templatedFile.split('.');
+    const template = fs
+      .readFileSync(`${templatedFileDesc[0]}.template.${templatedFileDesc[1]}`)
+      .toString();
+    const result = Handlebars.compile(template, {})(network);
+    fs.writeFileSync(`${templatedFileDesc[0]}.${templatedFileDesc[1]}`, result);
+  }
+
+  console.log(`🎉 subgraph successfully generated for ${networkName}\n`);
+})();