flutter_map WMS with custom CRS fails to load map image: Failed to decode Image data - wms

Question

I'm trying to create a simple flutter map widget that gets layers from a WMS server.
The server is this.
And you can see the its capabilities here.
Specifically, I want to use the layer "AMS_1956-1957".
As this layer is served in the CRS EPSG:4258, I'm creating a custom CRS for this FlutterMap to use (as it's stated in the documentation that only WGS84 (EPSG:4326) and Google Mercator (EPSG:3857) projections are supported).
I'm creating this custom CRS following the instructions here.
I'm getting the Proj4 definition string for this CRS (EPSG:4258) from here, as stated in the documentation: "+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs"
So the code for creating the custom CRS is this:
var resolutions = <double>[32768, 16384, 8192, 4096, 2048, 1024, 512, 256, 128];
var maxZoom = (resolutions.length - 1).toDouble();
var epsg4258CRS = Proj4Crs.fromFactory(
code: 'EPSG:4258',
proj4Projection: proj4.Projection.add("EPSG:4258",
'+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs'),
resolutions: resolutions,
);
Then I'm using this CRS in my FlutterMap widget as follows (again, based in the documentation).
#override
Widget build(BuildContext context) {
return FlutterMap(
options: MapOptions(
center: LatLng(41.61, -2.52),
zoom: 3.0,
),
layers: [
TileLayerOptions(
wmsOptions: WMSTileLayerOptions(
baseUrl: 'https://www.ign.es/wms/pnoa-historico?',
layers: ['AMS_1956-1957'],
crs: epsg4258CRS,
),
),
MarkerLayerOptions(
markers: [
Marker(
width: 5.0,
height: 5.0,
point: LatLng(41.61, -2.52),
builder: (ctx) => Container(
child: const FlutterLogo(),
),
),
],
),
],
);
}
}
The pointer is shown, so I guess it's doing something, but no map image is shown, and it returns the following error:
ImageCodecException: Failed to decode image data. Image source:
https://www.ign.es/wms/pnoa-historico?&service=WMS&request=GetMap&layers=AMS_1956-1957&styles=&format=image%2Fpng&srs=EPSG%3A4258&version=1.1.1&transparent=true&width=256&height=256&bbox=180,-90,180,-90
Ok, the WMS is working, as I can access it via QGIS, for example.
If I open the "image source" url created by flutter via browser it doesn't seem to work.
Plus, the bbox values in the url doesn't seem to make much sense, as they lie outside of the CRS boundaries. And the centering and zoom values I'm giving it doesn't seem to make any effect either...
If I try to open the url with different bbox values closer to the boundaries of the layer it shows something at least:
https://www.ign.es/wms/pnoa-historico?&service=WMS&request=GetMap&layers=AMS_1956-1957&styles=&format=image%2Fpng&srs=EPSG%3A4258&version=1.1.1&transparent=true&width=256&height=256&bbox=-9.5,2.5,12,44
Although I don't really fully understand what these numbers refer to (max x, min x, max y, min y?? // top-left corner, bottom-right corner // etc. ??)
Any help on what might be happening here will be much appreciated, thank you!