diff -r f2a40bbc27f6 -r eadaf0b8f02e server/src/app/Libraries/Mergers/CocoonAbstractRdfMerger.php
--- a/server/src/app/Libraries/Mergers/CocoonAbstractRdfMerger.php	Tue Nov 17 13:11:55 2015 +0100
+++ b/server/src/app/Libraries/Mergers/CocoonAbstractRdfMerger.php	Fri Nov 27 17:59:36 2015 +0100
@@ -26,10 +26,11 @@
      *
      * @return EasyRdf\Graph The new merged graph
      */
-    function mergeGraph($baseGraph, $srcGraph) {
+    function mergeGraph($baseGraph, $srcGraph, $uri = null) {
 
+        $uri = is_null($uri)?$baseGraph->getUri():$uri;
         $this->mergedArray = [];
-        $this->resGraph = new Graph($baseGraph->getUri());
+        $this->resGraph = new Graph($uri);
         $this->bnodeMerge = [];
         $this->baseGraph = $baseGraph;
         $this->srcGraph = $srcGraph;
@@ -78,11 +79,13 @@
                 $baseResource = $baseGraph->resource($baseUrl);
                 $srcResource = $srcGraph->resource($srcUrl);
 
-                $mergeMethod = $typeMergeMethodMap[$baseResource->typeAsResource()->getUri()];
-                $this->mergedArray[$bnodeId] = [];
+                if(!is_null($baseResource->typeAsResource()) && array_key_exists($baseResource->typeAsResource()->getUri(), $typeMergeMethodMap)) {
 
-                call_user_func(array($this, $mergeMethod), $baseResource, $srcResource, $bnodeId);
+                    $mergeMethod = $typeMergeMethodMap[$baseResource->typeAsResource()->getUri()];
+                    $this->mergedArray[$bnodeId] = [];
 
+                    call_user_func(array($this, $mergeMethod), $baseResource, $srcResource, $bnodeId);
+                }
             }
 
         }
@@ -183,8 +186,10 @@
 
 
     protected function mergeProperties($singleBNodeProperties, $singleProperties, &$targetArray, $baseRes, $srcRes) {
-        $srcArray = $this->srcGraph->toRdfPhp()[$srcRes->getUri()];
-        $baseArray = $this->baseGraph->toRdfPhp()[$baseRes->getUri()];
+        $srcRdfPhp = $this->srcGraph->toRdfPhp();
+        $srcArray = array_key_exists($srcRes->getUri(), $srcRdfPhp)?$srcRdfPhp[$srcRes->getUri()]:[];
+        $baseRdfPhp = $this->baseGraph->toRdfPhp();
+        $baseArray = array_key_exists($baseRes->getUri(), $baseRdfPhp)?$baseRdfPhp[$baseRes->getUri()]:[];
         foreach($srcRes->propertyUris() as $prop) {
             if(in_array($prop, $singleBNodeProperties)) {
                 $this->mergePropertySingleBNode($prop, $targetArray, $baseArray, $srcArray);