diff --git a/Module.cc b/Module.cc
index 923f5b1db..75e1c0f8d 100644
--- a/Module.cc
+++ b/Module.cc
@@ -139,3 +139,19 @@ PNamedItem::SymbolType Module::symbol_type() const
 
       return MODULE;
 }
+
+bool Module::can_be_toplevel() const
+{
+      // Don't choose library modules.
+      if (library_flag)
+	    return false;
+
+      // Don't choose modules with parameters without default value
+      for (std::map<perm_string,param_expr_t*>::const_iterator cur =
+	    parameters.begin(); cur != parameters.end(); cur++) {
+	    if (cur->second->expr == 0)
+		  return false;
+      }
+
+      return true;
+}
diff --git a/Module.h b/Module.h
index 3abcff0d1..3630d4b1c 100644
--- a/Module.h
+++ b/Module.h
@@ -166,6 +166,8 @@ class Module : public PScopeExtra, public PNamedItem {
 
       SymbolType symbol_type() const;
 
+      bool can_be_toplevel() const;
+
     private:
       void dump_specparams_(std::ostream&out, unsigned indent) const;
       std::list<PGate*> gates_;
diff --git a/main.cc b/main.cc
index ecedbb414..0c20b15cc 100644
--- a/main.cc
+++ b/main.cc
@@ -1187,8 +1187,7 @@ int main(int argc, char*argv[])
 	    for (mod = pform_modules.begin()
 		       ; mod != pform_modules.end() ; ++ mod ) {
 
-		    /* Don't choose library modules. */
-		  if ((*mod).second->library_flag)
+		  if (!(*mod).second->can_be_toplevel())
 			continue;
 
 		    /* Don't choose modules instantiated in other
diff --git a/net_design.cc b/net_design.cc
index 12c8b14c6..8112f2039 100644
--- a/net_design.cc
+++ b/net_design.cc
@@ -827,9 +827,19 @@ void NetScope::evaluate_parameter_(Design*des, param_ref_t cur)
       }
 
       // If the parameter has already been evaluated, quietly return.
-      if (cur->second.val_expr == 0)
+      if (cur->second.val != 0)
             return;
 
+      if (cur->second.val_expr == 0) {
+	    cerr << this->get_fileline() << ": error: "
+	         << "Missing value for parameter `"
+	         << cur->first << "`." << endl;
+	    des->errors += 1;
+
+	    cur->second.val = new NetEConst(verinum(verinum::Vx));
+	    return;
+      }
+
       // Guess the varaiable type of the parameter. If the parameter has no
       // given type, then guess the type from the expression and use that to
       // evaluate.
diff --git a/parse.y b/parse.y
index 9b98ca4f6..0129b515e 100644
--- a/parse.y
+++ b/parse.y
@@ -4901,7 +4901,11 @@ module_port_list_opt
      that the port declarations may use them. */
 module_parameter_port_list_opt
   :
-  | '#' '(' module_parameter_port_list ')'
+  | '#' '('
+    { pform_start_parameter_port_list(); }
+    module_parameter_port_list
+    { pform_end_parameter_port_list(); }
+    ')'
   ;
 
 module_parameter
@@ -5580,7 +5584,12 @@ parameter_assign_list
   ;
 
 parameter_assign
-  : IDENTIFIER '=' expression parameter_value_ranges_opt
+  : IDENTIFIER parameter_value_ranges_opt
+      { pform_set_parameter(@1, lex_strings.make($1), param_is_local,
+			    param_data_type, 0, $2);
+	delete[]$1;
+      }
+  | IDENTIFIER '=' expression parameter_value_ranges_opt
       { PExpr*tmp = $3;
 	pform_set_parameter(@1, lex_strings.make($1), param_is_local,
 			    param_data_type, tmp, $4);
diff --git a/pform.cc b/pform.cc
index 4ab111ea0..dd171773c 100644
--- a/pform.cc
+++ b/pform.cc
@@ -386,6 +386,9 @@ static unsigned pform_timescale_line;
 bool allow_timeunit_decl = true;
 bool allow_timeprec_decl = true;
 
+// Track whether the current parameter declaration is in a parameter port list
+static bool pform_in_parameter_port_list = false;
+
 static inline void FILE_NAME(LineInfo*obj, const char*file, unsigned lineno)
 {
       obj->set_lineno(lineno);
@@ -1378,6 +1381,16 @@ void pform_startmodule(const struct vlltype&loc, const char*name,
       pform_bind_attributes(cur_module->attributes, attr);
 }
 
+void pform_start_parameter_port_list()
+{
+      pform_in_parameter_port_list = true;
+}
+
+void pform_end_parameter_port_list()
+{
+      pform_in_parameter_port_list = false;
+}
+
 /*
  * This function is called by the parser to make a simple port
  * reference. This is a name without a .X(...), so the internal name
@@ -3228,6 +3241,17 @@ void pform_set_parameter(const struct vlltype&loc,
 	    return;
       }
 
+      if (expr == 0) {
+	    if (is_local) {
+		  VLerror(loc, "error: localparam must have a value.");
+	    } else if (!pform_in_parameter_port_list) {
+		  VLerror(loc, "error: parameter declared outside parameter "
+			        "port list must have a default value.");
+	    } else {
+		  pform_requires_sv(loc, "parameter without default value");
+	    }
+      }
+
       bool overridable = !is_local;
 
       if (scope == pform_cur_generate && !is_local) {
@@ -3240,7 +3264,6 @@ void pform_set_parameter(const struct vlltype&loc,
 	    overridable = false;
       }
 
-      assert(expr);
       Module::param_expr_t*parm = new Module::param_expr_t();
       FILE_NAME(parm, loc);
 
diff --git a/pform.h b/pform.h
index b1b5e7a1d..52199eaad 100644
--- a/pform.h
+++ b/pform.h
@@ -614,4 +614,7 @@ void pform_put_enum_type_in_scope(enum_type_t*enum_set);
 
 bool pform_requires_sv(const struct vlltype&loc, const char *feature);
 
+void pform_start_parameter_port_list();
+void pform_end_parameter_port_list();
+
 #endif /* IVL_pform_H */
diff --git a/pform_dump.cc b/pform_dump.cc
index 2aa8c8ecc..9367ce0d3 100644
--- a/pform_dump.cc
+++ b/pform_dump.cc
@@ -1485,11 +1485,9 @@ void LexicalScope::dump_parameters_(ostream&out, unsigned indent) const
 	          cur->second->data_type->debug_dump(out);
 	    else
 		  out << "(nil type)";
-	    out << " " << (*cur).first << " = ";
 	    if ((*cur).second->expr)
-		  out << *(*cur).second->expr;
-	    else
-		  out << "/* ERROR */";
+		  out << " " << (*cur).first << " = "
+		      << *(*cur).second->expr;
 	    for (LexicalScope::range_t*tmp = (*cur).second->range
 		       ; tmp ; tmp = tmp->next) {
 		  if (tmp->exclude_flag)