diff --git a/modules/io/src/mol/pdb_writer.cc b/modules/io/src/mol/pdb_writer.cc
index f46f70d040aa473edf037d443835edd24450096e..c37b7ce4fb9f526e5fe300329b914ae1288975a0 100644
--- a/modules/io/src/mol/pdb_writer.cc
+++ b/modules/io/src/mol/pdb_writer.cc
@@ -41,7 +41,7 @@ bool shift_left(const String& atom_name, bool is_hetatm,
     return false;
   }
 
-  if (isnumber(atom_name[0]) || atom_name=="UNK" ||
+  if (isdigit(atom_name[0]) || atom_name=="UNK" ||
       atom_name.length()==4) {
     return true;
   }