diff --git a/src/eval.c b/src/eval.c
index 1daf042..b82a98f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -98,6 +98,7 @@
 static char *e_listdictarg = N_("E712: Argument of %s must be a List or Dictionary");
 static char *e_listreq = N_("E714: List required");
 static char *e_dictreq = N_("E715: Dictionary required");
+static char *e_stringreq = N_("E928: String required");
 static char *e_toomanyarg = N_("E118: Too many arguments for function: %s");
 static char *e_dictkey = N_("E716: Key not present in Dictionary: %s");
 static char *e_funcexts = N_("E122: Function %s already exists, add ! to replace it");
@@ -18280,8 +18281,9 @@
     typval_T	*rettv)
 {
 #ifdef FEAT_QUICKFIX
+    static char *e_invact = N_("E927: Invalid action: '%s'");
     char_u	*act;
-    int		action = ' ';
+    int		action = 0;
 #endif
 
     rettv->vval.v_number = -1;
@@ -18298,11 +18300,17 @@
 	    act = get_tv_string_chk(action_arg);
 	    if (act == NULL)
 		return;		/* type error; errmsg already given */
-	    if (*act == 'a' || *act == 'r')
+	    if ((*act == 'a' || *act == 'r' || *act == ' ') && act[1] == NUL)
 		action = *act;
+	    else
+		EMSG2(_(e_invact), act);
 	}
+	else if (action_arg->v_type == VAR_UNKNOWN)
+	    action = ' ';
+	else
+	    EMSG(_(e_stringreq));
 
-	if (l != NULL && set_errorlist(wp, l, action,
+	if (l != NULL && action && set_errorlist(wp, l, action,
 	       (char_u *)(wp == NULL ? "setqflist()" : "setloclist()")) == OK)
 	    rettv->vval.v_number = 0;
     }