diff --git a/htdocs/core/class/hookmanager.class.php b/htdocs/core/class/hookmanager.class.php
index bd13cc238b1..d475060e9e5 100644
--- a/htdocs/core/class/hookmanager.class.php
+++ b/htdocs/core/class/hookmanager.class.php
@@ -131,12 +131,12 @@ class HookManager
         // Define type of hook ('output', 'returnvalue' or 'addreplace'). 'addreplace' should be type for all hooks. 'output' and 'returnvalue' are deprecated.
         $hooktype='output';
         if (preg_match('/^pdf_/',$method)) $hooktype='returnvalue';	// pdf_xxx except pdf_writelinedesc are returnvalue hooks. When there is 2 hooks of this type, only last one win.
-        if (in_array($method,array('addStatisticLine','doActions','formObjectOptions','moveUploadedFile','pdf_writelinedesc','paymentsupplierinvoices',
-        							'printSearchForm', 'formattachOptions', 'formBuilddocLineOptions'))) $hooktype='addreplace';
+        if (in_array($method,array('addStatisticLine','doActions','formObjectOptions','formattachOptions','formBuilddocLineOptions','moveUploadedFile','pdf_writelinedesc','paymentsupplierinvoices',
+        							'printSearchForm'))) $hooktype='addreplace';
 
         // Loop on each hook to qualify modules that declared context
         $modulealreadyexecuted=array();
-        $resaction=0; $error=0;
+        $resaction=0; $error=0; $result='';
 		$this->resPrint=''; $this->resArray=array();
         foreach($this->hooks as $modules)    // this->hooks is an array with context as key and value is an array of modules that handle this context
         {
@@ -144,7 +144,7 @@ class HookManager
             {
                 foreach($modules as $module => $actionclassinstance)
                 {
-                	//print "Before hook ".get_class($actionclassinstance)." method=".$method." results=".count($actionclassinstance->results)." resprints=".count($actionclassinstance->resprints)." result=".$result." resaction=".$resaction."<br>\n";
+                	//print "Before hook ".get_class($actionclassinstance)." method=".$method." hooktype=".$hooktype." results=".count($actionclassinstance->results)." resprints=".count($actionclassinstance->resprints)." resaction=".$resaction." result=".$result."<br>\n";
 
                 	//print 'class='.get_class($actionclassinstance).' method='.$method.' action='.$action;
                 	// jump to next class if method does not exists
@@ -160,7 +160,7 @@ class HookManager
                     	{
                     		$error++;
                     		$this->error=$actionclassinstance->error; $this->errors=array_merge($this->errors, (array) $actionclassinstance->errors);
-                    		// TODO remove this.
+                    		// TODO dead code to remove (do not enable this, but fix hook instead)
                     		/* Change must be inside the method of hook if required. Only hook must decide if $action must be modified or not.
                     		if ($method == 'doActions')
                     		{
@@ -172,7 +172,7 @@ class HookManager
                     	if (is_array($actionclassinstance->results))  $this->resArray =array_merge($this->resArray, $actionclassinstance->results);
                     	if (! empty($actionclassinstance->resprints)) $this->resPrint.=$actionclassinstance->resprints;
                     }
-                    // Generic hooks that return a string or array (printSearchForm, printLeftBlock, printTopRightMenu, formAddObjectLine, formBuilddocOptions, ...)
+                    // Generic hooks that return a string or array (printSearchForm, printLeftBlock, formAddObjectLine, formBuilddocOptions, ...)
                     else
 					{
                     	// TODO. this should be done into the method of hook by returning nothing
@@ -182,13 +182,18 @@ class HookManager
 
                     	if (! empty($actionclassinstance->results) && is_array($actionclassinstance->results)) $this->resArray =array_merge($this->resArray, $actionclassinstance->results);
                     	if (! empty($actionclassinstance->resprints)) $this->resPrint.=$actionclassinstance->resprints;
-                    	// TODO. remove this. array result must be set into $actionclassinstance->results
+                    	// TODO dead code to remove (do not enable this, but fix hook instead)
                     	//if (is_array($result)) $this->resArray = array_merge($this->resArray, $result);
-                    	// TODO. remove this. result must not be a string. we must use $actionclassinstance->resprints to return a string
-                    	//if (! is_array($result) && ! is_numeric($result)) $this->resPrint.=$result;
+                    	// TODO dead code to remove (do not enable this, but fix hook instead): result must not be a string. we must use $actionclassinstance->resprints to return a string
+                    	if (! is_array($result) && ! is_numeric($result))
+                    	{
+                    		//print 'Error: Bug into module '.get_class($actionclassinstance).' hook must not return a string but an int and set string into ->resprints';
+                    		dol_syslog('Error: Bug into module '.get_class($actionclassinstance).' hook must not return a string but an int and set string into ->resprints', LOG_ERR);
+                    		if (empty($actionclassinstance->resprints)) { $this->resPrint.=$result; $result=0; }
+                    	}
                     }
 
-                    //print "After hook  ".get_class($actionclassinstance)." method=".$method." results=".count($actionclassinstance->results)." resprints=".count($actionclassinstance->resprints)." result=".$result." resaction=".$resaction."<br>\n";
+                    //print "After hook  ".get_class($actionclassinstance)." method=".$method." hooktype=".$hooktype." results=".count($actionclassinstance->results)." resprints=".count($actionclassinstance->resprints)." resaction=".$resaction." result=".$result."<br>\n";
 
                     unset($actionclassinstance->results);
                     unset($actionclassinstance->resprints);