diff --git a/PGenerate.h b/PGenerate.h
index 85b1e0b8f..20e73fac8 100644
--- a/PGenerate.h
+++ b/PGenerate.h
@@ -115,6 +115,7 @@ class PGenerate : public PNamedItem, public LexicalScope {
       SymbolType symbol_type() const;
 
     private:
+      void check_for_valid_genvar_value_(long value);
       bool generate_scope_loop_(Design*des, NetScope*container);
       bool generate_scope_condit_(Design*des, NetScope*container, bool else_flag);
       bool generate_scope_case_(Design*des, NetScope*container);
diff --git a/elab_scope.cc b/elab_scope.cc
index 926e62efb..64180707e 100644
--- a/elab_scope.cc
+++ b/elab_scope.cc
@@ -895,6 +895,18 @@ bool PGenerate::generate_scope(Design*des, NetScope*container)
       }
 }
 
+void PGenerate::check_for_valid_genvar_value_(long value)
+{
+      if (generation_flag < GN_VER2005 && value < 0) {
+	    cerr << get_fileline() << ": warning: A negative value (" << value
+		 << ") has been assigned to genvar '" << loop_index << "'."
+		 << endl;
+	    cerr << get_fileline() << ":        : This is illegal in "
+		    "Verilog-2001. Use at least -g2005 to remove this warning."
+		 << endl;
+      }
+}
+
 /*
  * This is the elaborate scope method for a generate loop.
  */
@@ -939,6 +951,7 @@ bool PGenerate::generate_scope_loop_(Design*des, NetScope*container)
       }
 
       genvar = init->value().as_long();
+      check_for_valid_genvar_value_(genvar);
       delete init_ex;
 
       if (debug_scopes)
@@ -1009,6 +1022,7 @@ bool PGenerate::generate_scope_loop_(Design*des, NetScope*container)
 		       << genvar << " to " << step->value().as_long() << endl;
 
 	    genvar = step->value().as_long();
+	    check_for_valid_genvar_value_(genvar);
 	    container->genvar_tmp_val = genvar;
 	    delete step;
 	    delete test_ex;