diff --git a/kratos.gid/scripts/Writing/WriteGeometries.tcl b/kratos.gid/scripts/Writing/WriteGeometries.tcl
index 1e613cb74..d9a44a970 100644
--- a/kratos.gid/scripts/Writing/WriteGeometries.tcl
+++ b/kratos.gid/scripts/Writing/WriteGeometries.tcl
@@ -29,7 +29,7 @@ proc write::printGeometryConnectivities {group etype nnodes} {
     # increase indent (allows folding in text editor)
     incr ::write::current_mdpa_indent_level
     # Prepare the formats dict
-    set formats [GetFormatDict $group 0 $nnodes]
+    set formats [GetFormatDict $group "" $nnodes]
     # Write the connectivities
     GiD_WriteCalculationFile connectivities $formats
     # decrease indent
diff --git a/kratos.gid/scripts/Writing/Writing.tcl b/kratos.gid/scripts/Writing/Writing.tcl
index 271f0ac9e..5cb98d93d 100644
--- a/kratos.gid/scripts/Writing/Writing.tcl
+++ b/kratos.gid/scripts/Writing/Writing.tcl
@@ -288,12 +288,18 @@ proc write::transformGroupName {groupid} {
 
 # Warning! Indentation must be set before calling here!
 proc write::GetFormatDict { groupid mid num} {
+    set s [mdpaIndent]
+
     variable formats_dict
     set id_f [dict get $formats_dict ID]
-    set mid_f [dict get $formats_dict MAT_ID]
 
-    set s [mdpaIndent]
-    set f "${s}$id_f [format $mid_f $mid] [string repeat "$id_f " $num]\n"
+    set mid_str ""
+    if {$mid ne ""} {
+        set mid_f [dict get $formats_dict MAT_ID]
+        set mid_str [format $mid_f $mid]
+    }
+    
+    set f "${s}$id_f $mid_str [string repeat "$id_f " $num]\n"
     return [dict create $groupid $f]
 }