diff --git a/src/ol/source/ImageMapGuide.js b/src/ol/source/ImageMapGuide.js
index 59eb0b5249..1068840ac5 100644
--- a/src/ol/source/ImageMapGuide.js
+++ b/src/ol/source/ImageMapGuide.js
@@ -173,7 +173,7 @@ ImageMapGuide.prototype.getImageLoadFunction = function() {
  * @param {number} dpi The display resolution.
  * @return {number} The computed map scale.
  */
-ImageMapGuide.getScale = function(extent, size, metersPerUnit, dpi) {
+function getScale(extent, size, metersPerUnit, dpi) {
   const mcsW = getWidth(extent);
   const mcsH = getHeight(extent);
   const devW = size[0];
@@ -184,7 +184,7 @@ ImageMapGuide.getScale = function(extent, size, metersPerUnit, dpi) {
   } else {
     return mcsH * metersPerUnit / (devH * mpp); // height limited
   }
-};
+}
 
 
 /**
@@ -207,7 +207,7 @@ ImageMapGuide.prototype.updateParams = function(params) {
  * @return {string} The mapagent map image request URL.
  */
 ImageMapGuide.prototype.getUrl = function(baseUrl, params, extent, size, projection) {
-  const scale = ImageMapGuide.getScale(extent, size,
+  const scale = getScale(extent, size,
     this.metersPerUnit_, this.displayDpi_);
   const center = getCenter(extent);
   const baseParams = {