diff --git a/libopenfpga/libarchopenfpga/src/read_xml_circuit_library.cpp b/libopenfpga/libarchopenfpga/src/read_xml_circuit_library.cpp
index 9fb61a654..0954a0086 100644
--- a/libopenfpga/libarchopenfpga/src/read_xml_circuit_library.cpp
+++ b/libopenfpga/libarchopenfpga/src/read_xml_circuit_library.cpp
@@ -459,7 +459,7 @@ void read_xml_output_mask(pugi::xml_node& xml_port,
     if (circuit_lib.port_size(port) != mask_vector.size()) {
       archfpga_throw(loc_data.filename_c_str(), loc_data.line(xml_port),
                      "Invalid lut_output_mask attribute '%s'! It must match the port size (=%lu)\n",
-                     output_mask_attr, circuit_lib.port_size(port));
+                     output_mask_attr.c_str(), circuit_lib.port_size(port));
     } 
   } else {
     /* By default, we give a mask vector covering each pin of the port */