diff --git a/lib/compiler.js b/lib/compiler.js
index 309dc05..dc08f6e 100644
--- a/lib/compiler.js
+++ b/lib/compiler.js
@@ -349,7 +349,7 @@ function findModelDefinitions(rootDir, sources) {
 
 function resolveAppPath(rootDir, relativePath, resolveOptions) {
   var resolvedPath = tryResolveAppPath(rootDir, relativePath, resolveOptions);
-  if (resolvedPath === undefined) {
+  if (resolvedPath === undefined && !resolveOptions.optional) {
     var err = new Error('Cannot resolve path "' + relativePath + '"');
     err.code = 'PATH_NOT_FOUND';
     throw err;
@@ -469,7 +469,18 @@ function buildMiddlewareInstructions(rootDir, config) {
         allConfigs = [allConfigs];
 
       allConfigs.forEach(function(config) {
-        var resolved = resolveMiddlewarePath(rootDir, middleware);
+        var resolved = resolveMiddlewarePath(rootDir, middleware, config);
+
+        // resolved.sourceFile will be false-y if an optional middleware
+        // is not resolvable.
+        // if a non-optional middleware is not resolvable, it will throw
+        // at resolveAppPath() and not reach here
+        if (!resolved.sourceFile) {
+          return console.log('Middleware "%s" not found: %s',
+            middleware,
+            resolved.optional
+          );
+        }
 
         var middlewareConfig = cloneDeep(config);
         middlewareConfig.phase = phase;
@@ -507,14 +518,19 @@ function buildMiddlewareInstructions(rootDir, config) {
   };
 }
 
-function resolveMiddlewarePath(rootDir, middleware) {
-  var resolved = {};
+function resolveMiddlewarePath(rootDir, middleware, config) {
+  var resolved = {
+    optional: !!config.optional
+  };
 
   var segments = middleware.split('#');
   var pathName = segments[0];
   var fragment = segments[1];
   var middlewarePath = pathName;
-  var opts = { strict: true };
+  var opts = {
+    strict: true,
+    optional: !!config.optional
+  };
 
   if (fragment) {
     resolved.fragment = fragment;
@@ -660,6 +676,9 @@ function fixFileExtension(filepath, files, onlyScriptsExportingFunction) {
 
 function resolveAppScriptPath(rootDir, relativePath, resolveOptions) {
   var resolvedPath = resolveAppPath(rootDir, relativePath, resolveOptions);
+  if (!resolvedPath) {
+    return false;
+  }
   var sourceDir = path.dirname(resolvedPath);
   var files = tryReadDir(sourceDir);
   var fixedFile = fixFileExtension(resolvedPath, files, false);
diff --git a/test/compiler.test.js b/test/compiler.test.js
index a0a3aad..4d11d8b 100644
--- a/test/compiler.test.js
+++ b/test/compiler.test.js
@@ -1564,6 +1564,20 @@ describe('compiler', function() {
         .to.throw(/path-does-not-exist/);
     });
 
+    it('does not fail when an optional middleware cannot be resolved',
+      function() {
+      appdir.writeConfigFileSync('middleware.json', {
+        final: {
+          'loopback/path-does-not-exist': {
+            optional: 'this middleware is optional'
+          }
+        }
+      });
+
+      expect(function() { boot.compile(appdir.PATH); })
+        .to.not.throw();
+    });
+
     it('fails when a module middleware fragment cannot be resolved',
       function() {
         appdir.writeConfigFileSync('middleware.json', {
@@ -1578,6 +1592,20 @@ describe('compiler', function() {
           .to.throw(/path-does-not-exist/);
       });
 
+    it('does not fail when an optional middleware fragment cannot be resolved',
+      function() {
+        appdir.writeConfigFileSync('middleware.json', {
+          final: {
+            'loopback#path-does-not-exist': {
+              optional: 'this middleware is optional'
+            }
+          }
+        });
+
+        expect(function() { boot.compile(appdir.PATH); })
+          .to.not.throw();
+      });
+
     it('resolves paths relatively to appRootDir', function() {
       appdir.writeFileSync('my-middleware.js', '');
       appdir.writeConfigFileSync('./middleware.json', {