diff --git a/.travis.yml b/.travis.yml
index 5575707..06d2664 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,7 @@ php:
   - 5.5
   - 5.6
   - 7.0
+  - 7.1
 
 install:
   - alias composer=composer\ -n && composer selfupdate
diff --git a/composer.json b/composer.json
index cc0fb25..c271665 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
   "license": "LGPL-3.0",
   "require": {
     "php": ">=5.5",
-    "scriptfusion/mapper": "^1|^0",
+    "scriptfusion/mapper": "^1",
     "scriptfusion/static-class": "^1",
     "scriptfusion/retry-error-handlers": "^1",
     "psr/cache": "^1",
diff --git a/src/Options/EncapsulatedOptions.php b/src/Options/EncapsulatedOptions.php
index 03dc621..d91a44c 100644
--- a/src/Options/EncapsulatedOptions.php
+++ b/src/Options/EncapsulatedOptions.php
@@ -20,6 +20,36 @@ final public function copy()
         return $this->options + $this->defaults;
     }
 
+    /**
+     * Merges the specified overriding options into this instance giving precedence to the overriding options. Only
+     * option values that have been explicitly set are merged; that is, defaults are not merged.
+     *
+     * @param EncapsulatedOptions $overridingOptions Overriding options.
+     */
+    final public function merge(EncapsulatedOptions $overridingOptions)
+    {
+        if (!$overridingOptions instanceof static) {
+            throw new MergeException('Cannot merge: options must be an instance of "' . get_class($this) . '".');
+        }
+
+        $this->options = $this->mergeOptions($this->options, $overridingOptions->options);
+    }
+
+    /**
+     * Merges the specified options with the specified overrides giving precedence to the overriding options.
+     *
+     * Overriding this method in a derived class allows it to implement a custom merging strategy.
+     *
+     * @param array $options Options of this instance.
+     * @param array $overrides Overriding options.
+     *
+     * @return array Merged options.
+     */
+    protected function mergeOptions(array $options, array $overrides)
+    {
+        return $overrides + $options;
+    }
+
     /**
      * Gets the value for the specified option name. Returns the specified
      * default value if option name is not set.
@@ -61,6 +91,10 @@ final protected function &getReference($option)
      */
     final protected function set($option, $value)
     {
+        if (is_object($value) || is_resource($value)) {
+            throw new \InvalidArgumentException('Value must not be an object or resource.');
+        }
+
         $this->options["$option"] = $value;
 
         return $this;
diff --git a/src/Options/MergeException.php b/src/Options/MergeException.php
new file mode 100644
index 0000000..e8ba07a
--- /dev/null
+++ b/src/Options/MergeException.php
@@ -0,0 +1,10 @@
+<?php
+namespace ScriptFUSION\Porter\Options;
+
+/**
+ * The exception that is thrown when options cannot be merged.
+ */
+final class MergeException extends \RuntimeException
+{
+    // Intentionall empty.
+}
diff --git a/test/Porter/Options/TestOptions.php b/test/Porter/Options/TestOptions.php
index 3bf7149..fec86f5 100644
--- a/test/Porter/Options/TestOptions.php
+++ b/test/Porter/Options/TestOptions.php
@@ -3,7 +3,7 @@
 
 use ScriptFUSION\Porter\Options\EncapsulatedOptions;
 
-final class TestOptions extends EncapsulatedOptions
+class TestOptions extends EncapsulatedOptions
 {
     public function __construct()
     {
diff --git a/test/Unit/Porter/Options/EncapsulatedOptionsTest.php b/test/Unit/Porter/Options/EncapsulatedOptionsTest.php
index d1145cb..00a22e0 100644
--- a/test/Unit/Porter/Options/EncapsulatedOptionsTest.php
+++ b/test/Unit/Porter/Options/EncapsulatedOptionsTest.php
@@ -1,6 +1,8 @@
 <?php
 namespace ScriptFUSIONTest\Unit\Porter\Options;
 
+use ScriptFUSION\Porter\Options\EncapsulatedOptions;
+use ScriptFUSION\Porter\Options\MergeException;
 use ScriptFUSIONTest\Porter\Options\TestOptions;
 
 final class EncapsulatedOptionsTest extends \PHPUnit_Framework_TestCase
@@ -25,6 +27,20 @@ public function testSet()
         self::assertSame('bar', $this->options->getFoo());
     }
 
+    public function testSetObject()
+    {
+        $this->setExpectedException(\InvalidArgumentException::class);
+
+        $this->options->setFoo($this->options);
+    }
+
+    public function testSetResource()
+    {
+        $this->setExpectedException(\InvalidArgumentException::class);
+
+        $this->options->setFoo(STDIN);
+    }
+
     public function testSetNullOverridesDefault()
     {
         $this->options->setFoo(null);
@@ -41,6 +57,53 @@ public function testCopy()
         self::assertSame(['foo' => 'bar'], $this->options->copy());
     }
 
+    public function testMerge()
+    {
+        $a = $this->options;
+        $b = (new TestOptions)->setFoo('bar');
+        $c = clone $a;
+
+        self::assertSame('foo', $a->getFoo());
+        self::assertSame('bar', $b->getFoo());
+        self::assertSame('foo', $c->getFoo());
+
+        // Merging in b sets c to 'bar'.
+        $c->merge($b);
+
+        self::assertSame('foo', $a->getFoo());
+        self::assertSame('bar', $b->getFoo());
+        self::assertSame('bar', $c->getFoo());
+
+        // Merging in a does not change the value of c because no options have been set explicitly for a.
+        $c->merge($a);
+
+        self::assertSame('foo', $a->getFoo());
+        self::assertSame('bar', $b->getFoo());
+        self::assertSame('bar', $c->getFoo());
+
+        // Merging in a sets c to 'foo' after it has been explicitly set for a.
+        $a->setFoo('foo');
+        $c->merge($a);
+
+        self::assertSame('foo', $a->getFoo());
+        self::assertSame('bar', $b->getFoo());
+        self::assertSame('foo', $c->getFoo());
+    }
+
+    public function testMergeDerivedClass()
+    {
+        $this->options->merge(\Mockery::mock(TestOptions::class));
+
+        // PHPUnit asserts no exception is thrown.
+    }
+
+    public function testMergeNonDerivedClass()
+    {
+        $this->setExpectedException(MergeException::class, TestOptions::class);
+
+        $this->options->merge(\Mockery::mock(EncapsulatedOptions::class));
+    }
+
     public function testGetReference()
     {
         $this->options->setFoo(['bar' => 'bar', 'baz' => 'baz']);