diff --git a/packages/jsii-diff/bin/jsii-diff.ts b/packages/jsii-diff/bin/jsii-diff.ts
index dc38701222..533da0bc32 100644
--- a/packages/jsii-diff/bin/jsii-diff.ts
+++ b/packages/jsii-diff/bin/jsii-diff.ts
@@ -4,7 +4,7 @@ import spec = require('jsii-spec');
 import log4js = require('log4js');
 import yargs = require('yargs');
 import { compareAssemblies } from '../lib';
-import { downloadNpmPackage } from '../lib/util';
+import { DownloadFailure, downloadNpmPackage, showDownloadFailure } from '../lib/util';
 import { VERSION } from '../lib/version';
 
 const LOG = log4js.getLogger('jsii-diff');
@@ -33,13 +33,24 @@ async function main(): Promise<number> {
   configureLog4js(argv.verbose);
 
   LOG.debug(`Loading original assembly from ${(argv as any).original}`);
-  const original = await loadAssembly((argv as any).original);
+  const loadOriginal = await loadAssembly((argv as any).original);
+  if (!loadOriginal.success) {
+    process.stderr.write(`Could not load '${loadOriginal.resolved}': ${showDownloadFailure(loadOriginal.reason)}. Skipping analysis\n`);
+    return 0;
+  }
 
   LOG.debug(`Loading updated assembly from ${(argv as any).updated}`);
-  const updated = await loadAssembly((argv as any).updated);
+  const loadUpdated = await loadAssembly((argv as any).updated);
+  if (!loadUpdated.success) {
+    process.stderr.write(`Could not load '${loadUpdated.resolved}': ${showDownloadFailure(loadUpdated.reason)}. Skipping analysis\n`);
+    return 0;
+  }
+
+  const original = loadOriginal.assembly;
+  const updated = loadUpdated.assembly;
 
   if (original.name !== updated.name) {
-    process.stderr.write(`Look like different assemblies: '${original.name}' vs '${updated.name}'. Comparing is probably pointless...`);
+    process.stderr.write(`Look like different assemblies: '${original.name}' vs '${updated.name}'. Comparing is probably pointless...\n`);
   }
 
   LOG.info(`Starting analysis`);
@@ -66,19 +77,29 @@ async function main(): Promise<number> {
 // Allow both npm:<package> (legacy) and npm://<package> (looks better)
 const NPM_REGEX = /^npm:(\/\/)?/;
 
-async function loadAssembly(name: string) {
+/**
+ * Load the indicated assembly from the given name
+ *
+ * Supports downloading from NPM as well as from file or directory.
+ */
+async function loadAssembly(requested: string): Promise<LoadAssemblyResult> {
+  let resolved = requested;
   try {
-    if (name.match(NPM_REGEX)) {
-      let pkg = name.replace(NPM_REGEX, '');
+    if (requested.match(NPM_REGEX)) {
+      let pkg = requested.replace(NPM_REGEX, '');
       if (!pkg) { pkg = await loadPackageNameFromAssembly(); }
 
-      // Put 'pkg' back into 'name' so any errors loading the assembly get a good source description
-      name = `npm://${pkg}`;
-      if (pkg.indexOf('@', 1) === -1) { name += '@latest'; }
+      resolved = `npm://${pkg}`;
+      if (pkg.indexOf('@', 1) === -1) { resolved += '@latest'; }
 
-      return await downloadNpmPackage(pkg, loadFromFilesystem);
+      const download = await downloadNpmPackage(pkg, loadFromFilesystem);
+      if (download.success) {
+        return { requested, resolved, success: true, assembly: download.result };
+      }
+      return { requested, resolved, success: false, reason: download.reason };
     } else {
-      return await loadFromFilesystem(name);
+      // We don't accept failure loading from the filesystem
+      return { requested, resolved, success: true, assembly: await loadFromFilesystem(requested) };
     }
   } catch (e) {
     // Prepend information about which assembly we've failed to load
@@ -86,7 +107,7 @@ async function loadAssembly(name: string) {
     // Look at the type of error. If it has a lot of lines (like validation errors
     // tend to do) log everything to the debug log and only show a couple
     const maxLines = 3;
-    const messageWithContext = `Error loading assembly '${name}': ${e.message}`;
+    const messageWithContext = `Error loading assembly '${resolved}': ${e.message}`;
     const errorLines = messageWithContext.split('\n');
     if (errorLines.length < maxLines) { throw new Error(messageWithContext); }
     for (const line of errorLines) {
@@ -96,6 +117,9 @@ async function loadAssembly(name: string) {
   }
 }
 
+type LoadAssemblyResult = { requested: string; resolved: string }
+    & ({ success: true; assembly: reflect.Assembly } | { success: false; reason: DownloadFailure });
+
 async function loadPackageNameFromAssembly(): Promise<string> {
   const JSII_ASSEMBLY_FILE = '.jsii';
   if (!await fs.pathExists(JSII_ASSEMBLY_FILE)) {
diff --git a/packages/jsii-diff/lib/util.ts b/packages/jsii-diff/lib/util.ts
index bbc81bbb93..86a74e5aeb 100644
--- a/packages/jsii-diff/lib/util.ts
+++ b/packages/jsii-diff/lib/util.ts
@@ -21,19 +21,57 @@ export async function inTempDir<T>(block: () => T | Promise<T>): Promise<T> {
   }
 }
 
-export async function downloadNpmPackage<T>(pkg: string, block: (dir: string) => Promise<T>): Promise<T> {
+export type DownloadFailure = 'no_such_package';
+
+export type NpmDownloadResult<T> = { success: true; result: T  } | { success: false; reason: DownloadFailure };
+
+export function showDownloadFailure(f: DownloadFailure) {
+  switch (f) {
+    case 'no_such_package': return 'NPM package does not exist';
+  }
+}
+
+export async function downloadNpmPackage<T>(pkg: string, block: (dir: string) => Promise<T>): Promise<NpmDownloadResult<T>> {
   return await inTempDir(async () => {
     LOG.info(`Fetching NPM package ${pkg}`);
 
-    // Need to install package and dependencies in order for jsii-reflect
-    // to not bork when it can find the dependencies.
-    await exec(`npm install --silent --prefix . ${pkg}`);
+    try {
+      // Need to install package and dependencies in order for jsii-reflect
+      // to not bork when it can find the dependencies.
+      await exec(`npm install --silent --prefix . ${pkg}`);
+    } catch (e) {
+      // If this fails, might be because the package doesn't exist
+      if (!isSubprocesFailedError(e)) { throw e; }
+      if (await npmPackageExists(pkg)) {
+        throw new Error(`NPM fetch failed: ${e}. Please try again.`);
+      }
+      LOG.warn(`NPM package ${pkg} does not exist.`);
+      return { success: false, reason: 'no_such_package' } as NpmDownloadResult<T>;
+    }
 
     const pkgDir = trimVersionString(pkg);
-    return await block(path.join(process.cwd(), 'node_modules', pkgDir));
+    return {
+      success: true,
+      result: await block(path.join(process.cwd(), 'node_modules', pkgDir))
+    } as NpmDownloadResult<T>;
  });
 }
 
+function isSubprocesFailedError(e: any) {
+  return e.code !== undefined && e.cmd !== undefined;
+}
+
+async function npmPackageExists(pkg: string): Promise<boolean> {
+  try {
+    LOG.info(`Checking existence of ${pkg}`);
+    await exec(`npm show --silent ${pkg}`);
+    return true;
+  } catch (e) {
+    if (!isSubprocesFailedError(e)) { throw e; }
+    return false;
+  }
+}
+
 /**
  * Trim an optional version string from an NPM package name
  */