CSS flex box with a fixed height still stretches [duplicate]

I'm trying to create a header with two columns that will fill to the parent container's height. I'm using display:flex for a responsive layout. When I set the header to have a fixed height, the two other sub-divs have a gap between them and the header.

div#container {    padding:20px;    background:#F1F1F1;    display: flex;    flex-flow:row wrap;    height:540px;}.header{width:100%; height:40px; background:#ccc;}.content {    width:150px;    background:#ddd;    padding:10px;    margin-left: 10px;}

Fiddle

EDIT

The duplicate question is a nice description of the flex properties, but I'm not seeing an example like this addressed in it.If the height:40px is removed from the header, the header div will stretch down to the other divs. When the header height is specified, a gap exists.