diff --git a/src/userfunc.c b/src/userfunc.c
index 9ac1274..f3e4757 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -224,7 +224,6 @@
     char_u	*p;
     int		c;
     int		any_default = FALSE;
-    char_u	*expr;
     char_u	*whitep = *argp;
 
     if (newargs != NULL)
@@ -302,6 +301,34 @@
 	    arg = p;
 	    while (ASCII_ISALNUM(*p) || *p == '_')
 		++p;
+	    char_u *argend = p;
+
+	    if (*skipwhite(p) == '=')
+	    {
+		char_u *defval = skipwhite(skipwhite(p) + 1);
+		if (STRNCMP(defval, "v:none", 6) != 0)
+		{
+		    semsg(_(e_constructor_default_value_must_be_vnone_str), p);
+		    goto err_ret;
+		}
+		any_default = TRUE;
+		p = defval + 6;
+
+		if (ga_grow(default_args, 1) == FAIL)
+		    goto err_ret;
+
+		char_u *expr = vim_strsave((char_u *)"v:none");
+		if (expr == NULL)
+		    goto err_ret;
+		((char_u **)(default_args->ga_data))
+						 [default_args->ga_len] = expr;
+		default_args->ga_len++;
+	    }
+	    else if (any_default)
+	    {
+		emsg(_(e_non_default_argument_follows_default_argument));
+		goto err_ret;
+	    }
 
 	    // TODO: check the argument is indeed a member
 	    if (newargs != NULL && ga_grow(newargs, 1) == FAIL)
@@ -309,7 +336,7 @@
 	    if (newargs != NULL)
 	    {
 		((char_u **)(newargs->ga_data))[newargs->ga_len] =
-						    vim_strnsave(arg, p - arg);
+					       vim_strnsave(arg, argend - arg);
 		newargs->ga_len++;
 
 		if (argtypes != NULL && ga_grow(argtypes, 1) == OK)
@@ -322,15 +349,22 @@
 		    if (ga_grow(newlines, 1) == OK)
 		    {
 			// "this.name = name"
-			int len = 5 + (p - arg) + 3 + (p - arg) + 1;
+			int len = 5 + (argend - arg) + 3 + (argend - arg) + 1;
+			if (any_default)
+			    len += 14 + 10;
 			char_u *assignment = alloc(len);
 			if (assignment != NULL)
 			{
-			    c = *p;
-			    *p = NUL;
-			    vim_snprintf((char *)assignment, len,
+			    c = *argend;
+			    *argend = NUL;
+			    if (any_default)
+				vim_snprintf((char *)assignment, len,
+						"ifargisset %d this.%s = %s",
+					   default_args->ga_len - 1, arg, arg);
+			    else
+				vim_snprintf((char *)assignment, len,
 						     "this.%s = %s", arg, arg);
-			    *p = c;
+			    *argend = c;
 			    ((char_u **)(newlines->ga_data))[
 					      newlines->ga_len++] = assignment;
 			}
@@ -361,7 +395,7 @@
 		// find the end of the expression (doesn't evaluate it)
 		any_default = TRUE;
 		p = skipwhite(np + 1);
-		expr = p;
+		char_u *expr = p;
 		if (eval1(&p, &rettv, NULL) != FAIL)
 		{
 		    if (!skip)