diff --git a/src/classes/ExtendedGraph.class.php b/src/classes/ExtendedGraph.class.php
index 46a967da..e16aaa0b 100644
--- a/src/classes/ExtendedGraph.class.php
+++ b/src/classes/ExtendedGraph.class.php
@@ -177,6 +177,10 @@ public function add_literal_triple($s, $p, $o, $lang = null, $dt = null) {
      * @return bool
      */
     private function _add_triple($s, $p, Array $o_info) {
+        // Make sure the subject is not an empty string
+        if($s === ""){
+            throw new \Tripod\Exceptions\Exception("The subject cannot be an empty string");
+        }
         // The value $o should already have been validated by this point
         // It's validation differs depending on whether it is a literal or resource
         // So just check the subject and predicate here...
diff --git a/src/mongo/MongoGraph.class.php b/src/mongo/MongoGraph.class.php
index ead47088..3cded4bc 100644
--- a/src/mongo/MongoGraph.class.php
+++ b/src/mongo/MongoGraph.class.php
@@ -143,6 +143,9 @@ private function add_tarray_to_index($tarray)
                 if(!isset($value['r']) || !$this->isValidTripleValue($value['r'])){
                     return;
                 }
+                if($value['r'] === ""){
+                    throw new \Tripod\Exceptions\Exception("The subject cannot be an empty string");
+                }
             }
         }
         $_i[$this->_labeller->qname_to_alias($tarray["_id"][_ID_RESOURCE])] = $predObjects;
diff --git a/test/unit/ExtendedGraphTest.php b/test/unit/ExtendedGraphTest.php
index 41134376..a97f9e96 100644
--- a/test/unit/ExtendedGraphTest.php
+++ b/test/unit/ExtendedGraphTest.php
@@ -89,6 +89,13 @@ public function addInvalidSubjectToLiteralResultsInNoTriple_Provider(){
         );
     }
 
+    public function testAddEmptySubjectToLiteralThrowsException()
+    {
+        $this->setExpectedException('\Tripod\Exceptions\Exception');
+        $graph = new ExtendedGraph();
+        $graph->add_literal_triple("", 'http://some/predicate', 'http://someplace.com');
+    }
+
     /**
      * @dataProvider addInvalidSubjectToLiteralResultsInNoTriple_Provider
      */
@@ -176,6 +183,13 @@ public function addInvalidSubjectToResourceResultsInNoTriple_Provider(){
         );
     }
 
+    public function testAddEmptySubjectToResourceThrowsException()
+    {
+        $this->setExpectedException('\Tripod\Exceptions\Exception');
+        $graph = new ExtendedGraph();
+        $graph->add_resource_triple("", 'http://some/predicate', 'http://someplace.com');
+    }
+
     /**
      * @dataProvider addInvalidSubjectToLiteralResultsInNoTriple_Provider
      */
diff --git a/test/unit/mongo/MongoGraphTest.php b/test/unit/mongo/MongoGraphTest.php
index 304027b7..512cac19 100644
--- a/test/unit/mongo/MongoGraphTest.php
+++ b/test/unit/mongo/MongoGraphTest.php
@@ -229,6 +229,22 @@ public function addTripodArrayContainingInvalidSubject_Provider(){
         );
     }
 
+    public function testAddTripodArrayContainingEmptySubject()
+    {
+        $this->setExpectedException('\Tripod\Exceptions\Exception');
+        $doc = array(
+            "_id"=>array("r"=>"", "c"=>"http://talisaspire.com/works/4d101f63c10a6-2"),
+            "_version"=>0,
+            "rdf:type"=>array(
+                array("l"=>"a Value"),
+            ),
+            "bibo:isbn13"=>array("l"=>"9211234567890"),
+        );
+
+        $g = new \Tripod\Mongo\MongoGraph();
+        $g->add_tripod_array($doc);
+    }
+
 
     public function testAddTripodArrayContainingValidResourceValues()
     {