diff --git a/examples/wms-image-custom-proj.js b/examples/wms-image-custom-proj.js
index 4548247b84..c916a16df5 100644
--- a/examples/wms-image-custom-proj.js
+++ b/examples/wms-image-custom-proj.js
@@ -28,7 +28,7 @@ var layers = [
         'LAYERS': 'ch.swisstopo.pixelkarte-farbe-pk1000.noscale',
         'FORMAT': 'image/jpeg'
       },
-      serverType: 'mapserver',
+      serverType: /** @type {ol.source.wms.ServerType} */ ('mapserver'),
       extent: extent
     })
   }),
@@ -43,7 +43,7 @@ var layers = [
             'National parks / geo.admin.ch</a>'
       })],
       params: {'LAYERS': 'ch.bafu.schutzgebiete-paerke_nationaler_bedeutung'},
-      serverType: 'mapserver',
+      serverType: /** @type {ol.source.wms.ServerType} */ ('mapserver'),
       extent: extent
     })
   })
diff --git a/examples/wms-image.js b/examples/wms-image.js
index dd495aac46..1e25cb53e2 100644
--- a/examples/wms-image.js
+++ b/examples/wms-image.js
@@ -15,7 +15,7 @@ var layers = [
     source: new ol.source.ImageWMS({
       url: 'http://demo.opengeo.org/geoserver/wms',
       params: {'LAYERS': 'topp:states'},
-      serverType: 'geoserver',
+      serverType: /** @type {ol.source.wms.ServerType} */ ('geoserver'),
       extent: [-13884991, 2870341, -7455066, 6338219]
     })
   })
diff --git a/examples/wms-no-proj.js b/examples/wms-no-proj.js
index 260961b6b6..545b16a464 100644
--- a/examples/wms-no-proj.js
+++ b/examples/wms-no-proj.js
@@ -37,7 +37,7 @@ var layers = [
       })],
       crossOrigin: 'anonymous',
       params: {'LAYERS': 'ch.bafu.schutzgebiete-paerke_nationaler_bedeutung'},
-      serverType: 'mapserver',
+      serverType: /** @type {ol.source.wms.ServerType} */ ('mapserver'),
       url: 'http://wms.geo.admin.ch/'
     })
   })
diff --git a/src/objectliterals.jsdoc b/src/objectliterals.jsdoc
index ea3509e0cb..8dc27ba5d6 100644
--- a/src/objectliterals.jsdoc
+++ b/src/objectliterals.jsdoc
@@ -543,9 +543,6 @@
  * @property {ol.Extent|undefined} extent Extent.
  * @property {boolean|undefined} hidpi Use the `ol.Map#devicePixelRatio` value when
  *     requesting the image from the remote server. Default is `true`.
- * @property {ol.source.wms.ServerType|undefined} serverType The type of the remote WMS
- *     server: `mapserver`, `geoserver` or `qgis`. Only needed if `hidpi` is `true`.
- *     Default is `undefined`.
  * @property {boolean|undefined} useOverlay If `true`, will use
  *     `GETDYNAMICMAPOVERLAYIMAGE`.
  * @property {ol.proj.ProjectionLike} projection Projection.
@@ -607,6 +604,9 @@
  * @property {ol.Extent|undefined} extent Extent.
  * @property {boolean|undefined} hidpi Use the `ol.Map#devicePixelRatio` value when
  *     requesting the image from the remote server. Default is `true`.
+ * @property {ol.source.wms.ServerType|undefined} serverType The type of the remote WMS
+ *     server: `mapserver`, `geoserver` or `qgis`. Only needed if `hidpi` is `true`.
+ *     Default is `undefined`.
  * @property {Object.<string,*>} params WMS request parameters. At least a
  *     `LAYERS` param is required. `STYLES` is `` by default. `VERSION` is
  *     `1.3.0` by default. `WIDTH`, `HEIGHT`, `BBOX` and `CRS` (`SRS` for WMS
diff --git a/src/ol/source/wmssource.exports b/src/ol/source/wmssource.exports
new file mode 100644
index 0000000000..b5669e8ec7
--- /dev/null
+++ b/src/ol/source/wmssource.exports
@@ -0,0 +1,4 @@
+@exportSymbol ol.source.wms.ServerType
+@exportProperty ol.source.wms.ServerType.GEOSERVER
+@exportProperty ol.source.wms.ServerType.MAPSERVER
+@exportProperty ol.source.wms.ServerType.QGIS
diff --git a/src/ol/source/wmssource.js b/src/ol/source/wmssource.js
index 630a0026b5..6dde4c7663 100644
--- a/src/ol/source/wmssource.js
+++ b/src/ol/source/wmssource.js
@@ -10,8 +10,8 @@ goog.require('goog.uri.utils');
  * @enum {string}
  */
 ol.source.wms.ServerType = {
-  MAPSERVER: 'mapserver',
   GEOSERVER: 'geoserver',
+  MAPSERVER: 'mapserver',
   QGIS: 'qgis'
 };
 
@@ -63,7 +63,7 @@ ol.source.wms.getDpiParam = function(serverType, pixelRatio) {
   if (serverType == ol.source.wms.ServerType.MAPSERVER) {
     param['MAP_RESOLUTION'] = 90 * pixelRatio;
   } else if (serverType == ol.source.wms.ServerType.GEOSERVER) {
-    param['FORMAT_OPTION'] = 'dpi:' + 90 * pixelRatio;
+    param['FORMAT_OPTIONS'] = 'dpi:' + 90 * pixelRatio;
   } else if (serverType == ol.source.wms.ServerType.QGIS) {
     param['DPI'] = 90 * pixelRatio;
   } else {