diff --git a/buffer.c b/buffer.c
index 0c751bd8..1036f8ff 100644
--- a/buffer.c
+++ b/buffer.c
@@ -2734,14 +2734,21 @@ evbuffer_peek(struct evbuffer *buffer, ev_ssize_t len,
 		chain = buffer->first;
 	}
 
+	if (n_vec == 0 && len < 0) {
+		/* If no vectors are provided and they asked for "everything",
+		 * pretend they asked for the actual available amount. */
+		len = buffer->total_len - len_so_far;
+	}
+
 	while (chain) {
 		if (len >= 0 && len_so_far >= len)
 			break;
 		if (idx<n_vec) {
 			vec[idx].iov_base = chain->buffer + chain->misalign;
 			vec[idx].iov_len = chain->off;
-		} else if (len<0)
+		} else if (len<0) {
 			break;
+		}
 		++idx;
 		len_so_far += chain->off;
 		chain = chain->next;
diff --git a/include/event2/buffer.h b/include/event2/buffer.h
index f99e9a5b..895982e5 100644
--- a/include/event2/buffer.h
+++ b/include/event2/buffer.h
@@ -779,8 +779,10 @@ struct evbuffer_ptr evbuffer_search_eol(struct evbuffer *buffer,
     the buffer does not have as much data as you asked to see).
 
     @param buffer the evbuffer to peek into,
-    @param len the number of bytes to try to peek.  If negative, we
-       will try to fill as much of vec_out as we can.
+    @param len the number of bytes to try to peek.  If len is negative, we
+       will try to fill as much of vec_out as we can.  If len is negative
+       and vec_out is not provided, we return the number of evbuffer_iovecs
+       that would be needed to get all the data in the buffer.
     @param start_at an evbuffer_ptr indicating the point at which we
        should start looking for data.  NULL means, "At the start of the
        buffer."
diff --git a/test/regress_buffer.c b/test/regress_buffer.c
index 72e4dc54..ba9a1854 100644
--- a/test/regress_buffer.c
+++ b/test/regress_buffer.c
@@ -1780,6 +1780,10 @@ test_evbuffer_peek(void *info)
 		evbuffer_add_buffer(buf, tmp_buf);
 	}
 
+	/* How many chunks do we need for everything? */
+	i = evbuffer_peek(buf, -1, NULL, NULL, 0);
+	tt_int_op(i, ==, 16);
+
 	/* Simple peek: get everything. */
 	i = evbuffer_peek(buf, -1, NULL, v, 20);
 	tt_int_op(i, ==, 16); /* we used only 16 chunks. */