Index: src/app/legend/qgslegendlayerfile.cpp
===================================================================
--- src/app/legend/qgslegendlayerfile.cpp	(revision 7717)
+++ src/app/legend/qgslegendlayerfile.cpp	(working copy)
@@ -405,6 +405,10 @@
     case QgsVectorFileWriter::ErrCreateLayer:
       QMessageBox::warning(0, tr("Error"), tr("Layer creation failed"));
       break;
+    case QgsVectorFileWriter::ErrAttributeTypeUnsupported:
+      QMessageBox::warning(0, tr("Error"), 
+          tr("Layer attribute table contains unsupported datatype(s)"));
+      break;
   }
 }
 
Index: src/core/qgsvectorfilewriter.cpp
===================================================================
--- src/core/qgsvectorfilewriter.cpp	(revision 7717)
+++ src/core/qgsvectorfilewriter.cpp	(working copy)
@@ -122,7 +122,9 @@
         ogrType = OFTReal;
         break;
       default:
-        assert(0 && "invalid variant type!");
+        //assert(0 && "invalid variant type!");
+        mError = ErrAttributeTypeUnsupported;
+        return;
     }
 
     // create field definition
@@ -220,7 +222,8 @@
         poFeature->SetField(encAttrName.data(), mCodec->fromUnicode(attrValue.toString()).data());
         break;
       default:
-        assert(0 && "invalid variant type");
+        //assert(0 && "invalid variant type");
+        return false;
     }
   }
   
Index: src/core/qgsvectorfilewriter.h
===================================================================
--- src/core/qgsvectorfilewriter.h	(revision 7717)
+++ src/core/qgsvectorfilewriter.h	(working copy)
@@ -47,7 +47,8 @@
       NoError = 0,
       ErrDriverNotFound,
       ErrCreateDataSource,
-      ErrCreateLayer
+      ErrCreateLayer,
+      ErrAttributeTypeUnsupported
     };
 
     /** Write contents of vector layer to a shapefile */