diff --git a/fluent/src/builtins.js b/fluent/src/builtins.js
index ebd47b671..a91dda881 100644
--- a/fluent/src/builtins.js
+++ b/fluent/src/builtins.js
@@ -15,9 +15,9 @@ import { FluentNumber, FluentDateTime } from './types';
 
 export default {
   'NUMBER': ([arg], opts) =>
-    new FluentNumber(arg.value, merge(arg.opts, opts)),
+    new FluentNumber(value(arg), merge(arg.opts, opts)),
   'DATETIME': ([arg], opts) =>
-    new FluentDateTime(arg.value, merge(arg.opts, opts)),
+    new FluentDateTime(value(arg), merge(arg.opts, opts)),
 };
 
 function merge(argopts, opts) {
@@ -26,8 +26,15 @@ function merge(argopts, opts) {
 
 function values(opts) {
   const unwrapped = {};
-  for (const name of Object.keys(opts)) {
-    unwrapped[name] = opts[name].value;
+  for (const [name, opt] of Object.entries(opts)) {
+    unwrapped[name] = value(opt);
   }
   return unwrapped;
 }
+
+function value(arg) {
+  // StringExpression-typed options are parsed as regular strings by the
+  // runtime parser and are not converted to a FluentType by the resolver.
+  // They don't have the "value" property; they are the value.
+  return typeof arg === 'string' ? arg : arg.value;
+}
diff --git a/fluent/src/resolver.js b/fluent/src/resolver.js
index 02e4a9d99..473d9a2b5 100644
--- a/fluent/src/resolver.js
+++ b/fluent/src/resolver.js
@@ -457,7 +457,7 @@ function CallExpression(env, {fun, args}) {
   }
 
   const posargs = [];
-  const keyargs = [];
+  const keyargs = {};
 
   for (const arg of args) {
     if (arg.type === 'narg') {
@@ -467,8 +467,12 @@ function CallExpression(env, {fun, args}) {
     }
   }
 
-  // XXX functions should also report errors
-  return callee(posargs, keyargs);
+  try {
+    return callee(posargs, keyargs);
+  } catch (e) {
+    // XXX Report errors.
+    return FluentNone();
+  }
 }
 
 /**
diff --git a/fluent/src/types.js b/fluent/src/types.js
index d88dae3b8..e1f9430c7 100644
--- a/fluent/src/types.js
+++ b/fluent/src/types.js
@@ -54,10 +54,15 @@ export class FluentNumber extends FluentType {
   }
 
   valueOf(ctx) {
-    const nf = ctx._memoizeIntlObject(
-      Intl.NumberFormat, this.opts
-    );
-    return nf.format(this.value);
+    try {
+      const nf = ctx._memoizeIntlObject(
+        Intl.NumberFormat, this.opts
+      );
+      return nf.format(this.value);
+    } catch (e) {
+      // XXX Report the error.
+      return this.value;
+    }
   }
 
   /**
@@ -81,10 +86,15 @@ export class FluentDateTime extends FluentType {
   }
 
   valueOf(ctx) {
-    const dtf = ctx._memoizeIntlObject(
-      Intl.DateTimeFormat, this.opts
-    );
-    return dtf.format(this.value);
+    try {
+      const dtf = ctx._memoizeIntlObject(
+        Intl.DateTimeFormat, this.opts
+      );
+      return dtf.format(this.value);
+    } catch (e) {
+      // XXX Report the error.
+      return this.value;
+    }
   }
 }
 
diff --git a/fluent/test/functions_builtin_test.js b/fluent/test/functions_builtin_test.js
index 79ef00b92..fa9b5e501 100644
--- a/fluent/test/functions_builtin_test.js
+++ b/fluent/test/functions_builtin_test.js
@@ -6,46 +6,106 @@ import { MessageContext } from '../src/context';
 import { ftl } from './util';
 
 suite('Built-in functions', function() {
-  let ctx, args, errs;
-
-  setup(function() {
-    errs = [];
-  });
+  let ctx;
 
   suite('NUMBER', function(){
     suiteSetup(function() {
       ctx = new MessageContext('en-US', { useIsolating: false });
       ctx.addMessages(ftl`
-        foo = { NUMBER(1) }
+        num-decimal = { NUMBER($arg) }
+        num-percent = { NUMBER($arg, style: "percent") }
+        num-bad-opt = { NUMBER($arg, style: "bad") }
       `);
     });
 
-    test('formats the number', function() {
-      const msg = ctx.getMessage('foo');
-      const val = ctx.format(msg, args, errs);
-      assert.equal(val, '1');
-      assert.equal(errs.length, 0);
+    test('missing argument', function() {
+      let msg;
+
+      msg = ctx.getMessage('num-decimal');
+      assert.equal(ctx.format(msg), 'NaN');
+
+      msg = ctx.getMessage('num-percent');
+      assert.equal(ctx.format(msg), 'NaN');
+
+      msg = ctx.getMessage('num-bad-opt');
+      assert.equal(ctx.format(msg), 'NaN');
+    });
+
+    test('number argument', function() {
+      const args = {arg: 1};
+      let msg;
+
+      msg = ctx.getMessage('num-decimal');
+      assert.equal(ctx.format(msg, args), '1');
+
+      msg = ctx.getMessage('num-percent');
+      assert.equal(ctx.format(msg, args), '100%');
+
+      msg = ctx.getMessage('num-bad-opt');
+      assert.equal(ctx.format(msg, args), '1');
+    });
+
+    test('string argument', function() {
+      const args = {arg: "Foo"};
+      let msg;
+
+      msg = ctx.getMessage('num-decimal');
+      assert.equal(ctx.format(msg, args), 'NaN');
+
+      msg = ctx.getMessage('num-percent');
+      assert.equal(ctx.format(msg, args), 'NaN');
+
+      msg = ctx.getMessage('num-bad-opt');
+      assert.equal(ctx.format(msg, args), 'NaN');
     });
   });
 
   suite('DATETIME', function(){
-    let dtf;
-
     suiteSetup(function() {
-      dtf = new Intl.DateTimeFormat('en-US');
       ctx = new MessageContext('en-US', { useIsolating: false });
       ctx.addMessages(ftl`
-        foo = { DATETIME($date) }
+        dt-default = { DATETIME($arg) }
+        dt-month = { DATETIME($arg, month: "long") }
+        dt-bad-opt = { DATETIME($arg, month: "bad") }
       `);
     });
 
-    test('formats the date', function() {
+    test('missing argument', function() {
+      let msg;
+
+      msg = ctx.getMessage('dt-default');
+      assert.equal(ctx.format(msg), 'Invalid Date');
+
+      msg = ctx.getMessage('dt-month');
+      assert.equal(ctx.format(msg), 'Invalid Date');
+
+      msg = ctx.getMessage('dt-bad-opt');
+      assert.equal(ctx.format(msg), 'Invalid Date');
+    });
+
+    test.only('Date argument', function() {
       const date = new Date('2016-09-29');
-      const msg = ctx.getMessage('foo');
-      const val = ctx.format(msg, { date }, errs);
       // format the date argument to account for the testrunner's timezone
-      assert.equal(val, dtf.format(date));
-      assert.equal(errs.length, 0);
+      const expectedDefault =
+        (new Intl.DateTimeFormat('en-US')).format(date);
+      const expectedMonth =
+        (new Intl.DateTimeFormat('en-US', {month: 'long'})).format(date);
+
+      const args = {arg: date};
+      let msg;
+
+      msg = ctx.getMessage('dt-default');
+      assert.equal(ctx.format(msg, args), expectedDefault);
+
+      msg = ctx.getMessage('dt-month');
+      assert.equal(ctx.format(msg, args), expectedMonth);
+
+      msg = ctx.getMessage('dt-bad-opt');
+      // The argument value will be coerced into a string by the join operation
+      // in MessageContext.format.  The result looks something like this; it
+      // may vary depending on the TZ:
+      //     Thu Sep 29 2016 02:00:00 GMT+0200 (CEST)
+      assert.equal(ctx.format(msg, args), date.toString());
     });
   });
 });