JavaScript: what's wrong with this code? (Please, help!) : This error is driving me CRAZY. The function throws this error:

> Uncaught ReferenceError: Invalid left-hand side in assignmen*t

Could someone point out ***where*** is the damned error?

       function listo()
       {
    	   if (document.getElementById('titulo')='') {
    		   AbreSnackBar('Debes especificar un título para esta obra','rojo');
    		   exit();
    	   }
    	   
    		var data=
    		   'titulo=' + document.getElementById('titulo').value +
    		   '&plantilla=' + document.getElementById('plantilla').value +
    		   '&pags=' + document.getElementById('pags').value +
    		   '&genero=' + document.getElementById('gen').value +
    		   '&comp_real=' + document.getElementById('comp_orig').value +
    		   '&ano=' + document.getElementById('ano').value +
    		   '&up_im1=' + document.getElementById('up_im1').value +
    		   '&up_im2=' + document.getElementById('up_im2').value +
    		   '&up_im3=' + document.getElementById('up_im3').value +
    		   '&up_ref=' + document.getElementById('up_ref').value +
    		   '&video=' + document.getElementById('link').value +
    		   '&descr=' + document.getElementById('descr').value +
    		   '&descr_ing=' + document.getElementById('descr_ing').value +
    		   '&minutos=' + document.getElementById('minutos').value +
    		   '&dificultad=' + document.getElementById('dif').value +
    		   '&up_PDF=' + document.getElementById('up_PDF').value + 
    		   '&up_RAR=' + document.getElementById('up_RAR').value +
    		   '&up_MUS=' + document.getElementById('up_MUS').value +
    		   '&up_SIB=' + document.getElementById('up_SIB').value +
    		   //'&tags=' + document.getElementById('tags').value +
    		   '&modo=' + document.getElementById('modo').value +
    		   '&id_concurso=' + document.getElementById('id_concurso').value +
    		   '&copias_min=' + document.getElementById('copias_min').value;
    
    		AbreSnackBar('Espera...');
    		$.ajax({
    			type: "POST",		url: "acc_sube_obra.php", data: data ,
    			success: function (data) 
    			{
    				alert(data);
    				switch (data.trim())
    				{
    					case '0':
    						AbreSnackBar('Ocurrió un error','rojo');
    						break;
    					default: //id
    						AbreSnackBar('Bien','verde');
    						window.location.href = 'http://www.aboutscores.com/obra_subida.php?id='+ data.trim;
    				}
    			} 
    		});	
       }

I've seen that the cause could be that I'm *trying to assign a new value to the result of a function*, but I don't see that's the case.
