diff --git a/src/acl.c b/src/acl.c
index 193788c..e3fcf62 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -123,7 +123,8 @@
 /* Parse an ACL expression starting at <args>[0], and return it. If <err> is
  * not NULL, it will be filled with a pointer to an error message in case of
  * error. This pointer must be freeable or NULL. <al> is an arg_list serving
- * as a list head to report missing dependencies.
+ * as a list head to report missing dependencies. It may be NULL if such
+ * dependencies are not allowed.
  *
  * Right now, the only accepted syntax is :
  * <subject> [<value>...]
@@ -164,9 +165,11 @@
 	 * keyword.
 	 */
 
-	al->ctx  = ARGC_ACL;   // to report errors while resolving args late
-	al->kw   = *args;
-	al->conv = NULL;
+	if (al) {
+		al->ctx  = ARGC_ACL;   // to report errors while resolving args late
+		al->kw   = *args;
+		al->conv = NULL;
+	}
 
 	aclkw = find_acl_kw(args[0]);
 	if (aclkw) {
@@ -282,8 +285,10 @@
 			conv_expr->conv = conv;
 			acl_conv_found = 1;
 
-			al->kw = smp->fetch->kw;
-			al->conv = conv_expr->conv->kw;
+			if (al) {
+				al->kw = smp->fetch->kw;
+				al->conv = conv_expr->conv->kw;
+			}
 			argcnt = make_arg_list(endw, -1, conv->arg_mask, &conv_expr->arg_p, err, &arg, &err_arg, al);
 			if (argcnt < 0) {
 				memprintf(err, "ACL keyword '%s' : invalid arg %d in converter '%s' : %s.",
@@ -667,7 +672,7 @@
  * an anonymous one and it won't be merged with any other one. If <err> is not
  * NULL, it will be filled with an appropriate error. This pointer must be
  * freeable or NULL. <al> is the arg_list serving as a head for unresolved
- * dependencies.
+ * dependencies. It may be NULL if such dependencies are not allowed.
  *
  * args syntax: <aclname> <acl_expr>
  */
@@ -780,7 +785,8 @@
  * If <known_acl> is not NULL, the ACL will be queued at its tail. If <err> is
  * not NULL, it will be filled with an error message if an error occurs. This
  * pointer must be freeable or NULL. <al> is an arg_list serving as a list head
- * to report missing dependencies.
+ * to report missing dependencies. It may be NULL if such dependencies are not
+ * allowed.
  */
 static struct acl *find_acl_default(const char *acl_name, struct list *known_acl,
                                     char **err, struct arg_list *al,
@@ -860,7 +866,8 @@
  * <err> is not NULL, it will be filled with a pointer to an error message in
  * case of error, that the caller is responsible for freeing. The initial
  * location must either be freeable or NULL. The list <al> serves as a list head
- * for unresolved dependencies.
+ * for unresolved dependencies. It may be NULL if such dependencies are not
+ * allowed.
  */
 struct acl_cond *parse_acl_cond(const char **args, struct list *known_acl,
                                 enum acl_cond_pol pol, char **err, struct arg_list *al,
diff --git a/src/arg.c b/src/arg.c
index d44f268..4e40aae 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -96,14 +96,15 @@
  * The returned array is always terminated by an arg of type ARGT_STOP (0),
  * unless the mask indicates that no argument is supported. Unresolved arguments
  * are appended to arg list <al>, which also serves as a template to create new
- * entries. The mask is composed of a number of mandatory arguments in its lower
- * ARGM_BITS bits, and a concatenation of each argument type in each subsequent
- * ARGT_BITS-bit sblock. If <err_msg> is not NULL, it must point to a freeable
- * or NULL pointer. The caller is expected to restart the parsing from the new
- * pointer set in <end_ptr>, which is the first character considered as not
- * being part of the arg list. The input string ends on the first between <len>
- * characters (when len is positive) or the first NUL character. Placing -1 in
- * <len> will make it virtually unbounded (~2GB long strings).
+ * entries. <al> may be NULL if unresolved arguments are not allowed. The mask
+ * is composed of a number of mandatory arguments in its lower ARGM_BITS bits,
+ * and a concatenation of each argument type in each subsequent ARGT_BITS-bit
+ * sblock. If <err_msg> is not NULL, it must point to a freeable or NULL
+ * pointer. The caller is expected to restart the parsing from the new pointer
+ * set in <end_ptr>, which is the first character considered as not being part
+ * of the arg list. The input string ends on the first between <len> characters
+ * (when len is positive) or the first NUL character. Placing -1 in <len> will
+ * make it virtually unbounded (~2GB long strings).
  */
 int make_arg_list(const char *in, int len, uint64_t mask, struct arg **argp,
                   char **err_msg, const char **end_ptr, int *err_arg,
@@ -244,6 +245,8 @@
 			/* These argument types need to be stored as strings during
 			 * parsing then resolved later.
 			 */
+			if (!al)
+				goto resolve_err;
 			arg->unresolved = 1;
 			new_al = arg_list_add(al, arg, pos);
 
@@ -445,4 +448,9 @@
 alloc_err:
 	memprintf(err_msg, "out of memory");
 	goto err;
+
+ resolve_err:
+	memprintf(err_msg, "unresolved argument of type '%s' at position %d not allowed",
+	          arg_type_names[(mask >> (pos * ARGT_BITS)) & ARGT_MASK], pos + 1);
+	goto err;
 }
diff --git a/src/sample.c b/src/sample.c
index 075763a..c010580 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -861,7 +861,9 @@
  * Parse a sample expression configuration:
  *        fetch keyword followed by format conversion keywords.
  * Returns a pointer on allocated sample expression structure.
- * The caller must have set al->ctx.
+ * <al> is an arg_list serving as a list head to report missing dependencies.
+ * It may be NULL if such dependencies are not allowed. Otherwise, the caller
+ * must have set al->ctx if al is set.
  * If <endptr> is non-nul, it will be set to the first unparsed character
  * (which may be the final '\0') on success. If it is nul, the expression
  * must be properly terminated by a '\0' otherwise an error is reported.
@@ -920,8 +922,10 @@
 	 * this allows it to automatically create entries for mandatory
 	 * implicit arguments (eg: local proxy name).
 	 */
-	al->kw = expr->fetch->kw;
-	al->conv = NULL;
+	if (al) {
+		al->kw = expr->fetch->kw;
+		al->conv = NULL;
+	}
 	if (make_arg_list(endw, -1, fetch->arg_mask, &expr->arg_p, err_msg, &endt, &err_arg, al) < 0) {
 		memprintf(err_msg, "fetch method '%s' : %s", fkw, *err_msg);
 		goto out_error;
@@ -1021,8 +1025,10 @@
 		LIST_APPEND(&(expr->conv_exprs), &(conv_expr->list));
 		conv_expr->conv = conv;
 
-		al->kw = expr->fetch->kw;
-		al->conv = conv_expr->conv->kw;
+		if (al) {
+			al->kw = expr->fetch->kw;
+			al->conv = conv_expr->conv->kw;
+		}
 		argcnt = make_arg_list(endw, -1, conv->arg_mask, &conv_expr->arg_p, err_msg, &endt, &err_arg, al);
 		if (argcnt < 0) {
 			memprintf(err_msg, "invalid arg %d in converter '%s' : %s", err_arg+1, ckw, *err_msg);