diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index ac55d89e4..e341b18db 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -352,10 +352,10 @@ class ValidatingArrayLoader implements LoaderInterface
                 if ($srcType === 'source' && !isset($this->config[$srcType]['reference'])) {
                     $this->errors[] = $srcType . '.reference : must be present';
                 }
-                if (!is_string($this->config[$srcType]['type'])) {
+                if (isset($this->config[$srcType]['type']) && !is_string($this->config[$srcType]['type'])) {
                     $this->errors[] = $srcType . '.type : should be a string, '.gettype($this->config[$srcType]['type']).' given';
                 }
-                if (!is_string($this->config[$srcType]['url'])) {
+                if (isset($this->config[$srcType]['url']) && !is_string($this->config[$srcType]['url'])) {
                     $this->errors[] = $srcType . '.url : should be a string, '.gettype($this->config[$srcType]['url']).' given';
                 }
                 if (isset($this->config[$srcType]['reference']) && !is_string($this->config[$srcType]['reference']) && !is_int($this->config[$srcType]['reference'])) {
@@ -364,7 +364,7 @@ class ValidatingArrayLoader implements LoaderInterface
                 if (isset($this->config[$srcType]['reference']) && Preg::isMatch('{^\s*-}', (string) $this->config[$srcType]['reference'])) {
                     $this->errors[] = $srcType . '.reference : must not start with a "-", "'.$this->config[$srcType]['reference'].'" given';
                 }
-                if (Preg::isMatch('{^\s*-}', $this->config[$srcType]['url'])) {
+                if (isset($this->config[$srcType]['url']) && Preg::isMatch('{^\s*-}', (string) $this->config[$srcType]['url'])) {
                     $this->errors[] = $srcType . '.url : must not start with a "-", "'.$this->config[$srcType]['url'].'" given';
                 }
             }
diff --git a/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php b/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
index fd105f7e1..ad368bf24 100644
--- a/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
@@ -417,6 +417,20 @@ class ValidatingArrayLoaderTest extends TestCase
                     'require.foo/Bar : a package cannot set a require on itself',
                 ),
             ),
+            array(
+                array(
+                    'name' => 'foo/bar',
+                    'source' => array('url' => 1),
+                    'dist' => array('url' => null),
+                ),
+                array(
+                    'source.type : must be present',
+                    'source.url : should be a string, integer given',
+                    'source.reference : must be present',
+                    'dist.type : must be present',
+                    'dist.url : must be present',
+                ),
+            ),
         ));
     }