diff --git a/CHANGES.md b/CHANGES.md
index 23315b626327..05058e2b3329 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -68,6 +68,7 @@ Change Log
 * Improved rendering of glTF models that don't contain normals with a temporary unlit shader workaround. [#6501](https://github.com/AnalyticalGraphicsInc/cesium/pull/6501)
 * Fixed rendering of glTF models with emissive-only materials. [#6501](https://github.com/AnalyticalGraphicsInc/cesium/pull/6501)
 * Fixed a bug in shader modification for glTF 1.0 quantized attributes and Draco quantized attributes. [#6523](https://github.com/AnalyticalGraphicsInc/cesium/pull/6523)
+* Fixed a bug causing custom TilingScheme classes to not be able to use a GeographicProjection. [#6524](https://github.com/AnalyticalGraphicsInc/cesium/pull/6524)
 
 ### 1.44 - 2018-04-02
 
diff --git a/Source/Core/HeightmapTerrainData.js b/Source/Core/HeightmapTerrainData.js
index 44696bb5a478..905ece4de512 100644
--- a/Source/Core/HeightmapTerrainData.js
+++ b/Source/Core/HeightmapTerrainData.js
@@ -4,7 +4,7 @@ define([
         './defined',
         './defineProperties',
         './DeveloperError',
-        './GeographicTilingScheme',
+        './GeographicProjection',
         './HeightmapTessellator',
         './Math',
         './Rectangle',
@@ -18,7 +18,7 @@ define([
         defined,
         defineProperties,
         DeveloperError,
-        GeographicTilingScheme,
+        GeographicProjection,
         HeightmapTessellator,
         CesiumMath,
         Rectangle,
@@ -222,7 +222,7 @@ define([
             relativeToCenter : center,
             ellipsoid : ellipsoid,
             skirtHeight : this._skirtHeight,
-            isGeographic : tilingScheme instanceof GeographicTilingScheme,
+            isGeographic : tilingScheme.projection instanceof GeographicProjection,
             exaggeration : exaggeration
         });
 
diff --git a/Source/Scene/ArcGisMapServerImageryProvider.js b/Source/Scene/ArcGisMapServerImageryProvider.js
index 489d6d632abe..1a76dd8caabc 100644
--- a/Source/Scene/ArcGisMapServerImageryProvider.js
+++ b/Source/Scene/ArcGisMapServerImageryProvider.js
@@ -8,6 +8,7 @@ define([
         '../Core/defineProperties',
         '../Core/DeveloperError',
         '../Core/Event',
+        '../Core/GeographicProjection',
         '../Core/GeographicTilingScheme',
         '../Core/Math',
         '../Core/Rectangle',
@@ -30,6 +31,7 @@ define([
         defineProperties,
         DeveloperError,
         Event,
+        GeographicProjection,
         GeographicTilingScheme,
         CesiumMath,
         Rectangle,
@@ -259,7 +261,7 @@ define([
                 f: 'image'
             };
 
-            if (imageryProvider._tilingScheme instanceof GeographicTilingScheme) {
+            if (imageryProvider._tilingScheme.projection instanceof GeographicProjection) {
                 query.bboxSR = 4326;
                 query.imageSR = 4326;
             } else {
@@ -622,7 +624,7 @@ define([
         var horizontal;
         var vertical;
         var sr;
-        if (this._tilingScheme instanceof GeographicTilingScheme) {
+        if (this._tilingScheme.projection instanceof GeographicProjection) {
             horizontal = CesiumMath.toDegrees(longitude);
             vertical = CesiumMath.toDegrees(latitude);
             sr = '4326';
diff --git a/Source/Scene/ImageryLayer.js b/Source/Scene/ImageryLayer.js
index 02350f6e274f..5764b35f46f1 100644
--- a/Source/Scene/ImageryLayer.js
+++ b/Source/Scene/ImageryLayer.js
@@ -7,6 +7,7 @@ define([
         '../Core/destroyObject',
         '../Core/DeveloperError',
         '../Core/FeatureDetection',
+        '../Core/GeographicProjection',
         '../Core/GeographicTilingScheme',
         '../Core/IndexDatatype',
         '../Core/Math',
@@ -48,6 +49,7 @@ define([
         destroyObject,
         DeveloperError,
         FeatureDetection,
+        GeographicProjection,
         GeographicTilingScheme,
         IndexDatatype,
         CesiumMath,
@@ -480,7 +482,7 @@ define([
         // Use Web Mercator for our texture coordinate computations if this imagery layer uses
         // that projection and the terrain tile falls entirely inside the valid bounds of the
         // projection.
-        var useWebMercatorT = imageryProvider.tilingScheme instanceof WebMercatorTilingScheme &&
+        var useWebMercatorT = imageryProvider.tilingScheme.projection instanceof WebMercatorProjection &&
                               tile.rectangle.north < WebMercatorProjection.MaximumLatitude &&
                               tile.rectangle.south > -WebMercatorProjection.MaximumLatitude;
 
@@ -847,7 +849,7 @@ define([
             });
         }
 
-        if (imageryProvider.tilingScheme instanceof WebMercatorTilingScheme) {
+        if (imageryProvider.tilingScheme.projection instanceof WebMercatorProjection) {
             imagery.textureWebMercator = texture;
         } else {
             imagery.texture = texture;
@@ -932,7 +934,7 @@ define([
         // avoids precision problems in the reprojection transformation while making
         // no noticeable difference in the georeferencing of the image.
         if (needGeographicProjection &&
-            !(this._imageryProvider.tilingScheme instanceof GeographicTilingScheme) &&
+            !(this._imageryProvider.tilingScheme.projection instanceof GeographicProjection) &&
             rectangle.width / texture.width > 1e-5) {
                 var that = this;
                 imagery.addReference();
@@ -1210,7 +1212,7 @@ define([
         var imageryProvider = layer._imageryProvider;
         var tilingScheme = imageryProvider.tilingScheme;
         var ellipsoid = tilingScheme.ellipsoid;
-        var latitudeFactor = !(layer._imageryProvider.tilingScheme instanceof GeographicTilingScheme) ? Math.cos(latitudeClosestToEquator) : 1.0;
+        var latitudeFactor = !(layer._imageryProvider.tilingScheme.projection instanceof GeographicProjection) ? Math.cos(latitudeClosestToEquator) : 1.0;
         var tilingSchemeRectangle = tilingScheme.rectangle;
         var levelZeroMaximumTexelSpacing = ellipsoid.maximumRadius * tilingSchemeRectangle.width * latitudeFactor / (imageryProvider.tileWidth * tilingScheme.getNumberOfXTilesAtLevel(0));
 
diff --git a/Source/Scene/UrlTemplateImageryProvider.js b/Source/Scene/UrlTemplateImageryProvider.js
index 6fccf9d4cd04..0757f4822705 100644
--- a/Source/Scene/UrlTemplateImageryProvider.js
+++ b/Source/Scene/UrlTemplateImageryProvider.js
@@ -9,7 +9,7 @@ define([
         '../Core/defineProperties',
         '../Core/DeveloperError',
         '../Core/Event',
-        '../Core/GeographicTilingScheme',
+        '../Core/GeographicProjection',
         '../Core/isArray',
         '../Core/Math',
         '../Core/Rectangle',
@@ -28,7 +28,7 @@ define([
         defineProperties,
         DeveloperError,
         Event,
-        GeographicTilingScheme,
+        GeographicProjection,
         isArray,
         CesiumMath,
         Rectangle,
@@ -961,7 +961,7 @@ define([
             return;
         }
 
-        if (imageryProvider.tilingScheme instanceof GeographicTilingScheme) {
+        if (imageryProvider.tilingScheme.projection instanceof GeographicProjection) {
             longitudeLatitudeProjectedScratch.x = CesiumMath.toDegrees(longitude);
             longitudeLatitudeProjectedScratch.y = CesiumMath.toDegrees(latitude);
         } else {
diff --git a/Source/Scene/WebMapServiceImageryProvider.js b/Source/Scene/WebMapServiceImageryProvider.js
index 5dc05e2f2076..9e581c0b7908 100644
--- a/Source/Scene/WebMapServiceImageryProvider.js
+++ b/Source/Scene/WebMapServiceImageryProvider.js
@@ -6,7 +6,7 @@ define([
         '../Core/freezeObject',
         '../Core/GeographicTilingScheme',
         '../Core/Resource',
-        '../Core/WebMercatorTilingScheme',
+        '../Core/WebMercatorProjection',
         './GetFeatureInfoFormat',
         './UrlTemplateImageryProvider'
     ], function(
@@ -17,7 +17,7 @@ define([
         freezeObject,
         GeographicTilingScheme,
         Resource,
-        WebMercatorTilingScheme,
+        WebMercatorProjection,
         GetFeatureInfoFormat,
         UrlTemplateImageryProvider) {
     'use strict';
@@ -118,10 +118,10 @@ define([
             // Use CRS with 1.3.0 and going forward.
             // For GeographicTilingScheme, use CRS:84 vice EPSG:4326 to specify lon, lat (x, y) ordering for
             // bbox requests.
-            parameters.crs = defaultValue(options.crs, options.tilingScheme instanceof WebMercatorTilingScheme ? 'EPSG:3857' : 'CRS:84');
+            parameters.crs = defaultValue(options.crs, options.tilingScheme && options.tilingScheme.projection instanceof WebMercatorProjection ? 'EPSG:3857' : 'CRS:84');
         } else {
             // SRS for WMS 1.1.0 or 1.1.1.
-            parameters.srs = defaultValue(options.srs, options.tilingScheme instanceof WebMercatorTilingScheme ? 'EPSG:3857' : 'EPSG:4326');
+            parameters.srs = defaultValue(options.srs, options.tilingScheme && options.tilingScheme.projection instanceof WebMercatorProjection ? 'EPSG:3857' : 'EPSG:4326');
         }
 
         resource.setQueryParameters(parameters, true);
diff --git a/Source/Scene/createTileMapServiceImageryProvider.js b/Source/Scene/createTileMapServiceImageryProvider.js
index c9b433b888dd..28938467cdcb 100644
--- a/Source/Scene/createTileMapServiceImageryProvider.js
+++ b/Source/Scene/createTileMapServiceImageryProvider.js
@@ -4,6 +4,7 @@ define([
         '../Core/defaultValue',
         '../Core/defined',
         '../Core/DeveloperError',
+        '../Core/GeographicProjection',
         '../Core/GeographicTilingScheme',
         '../Core/Rectangle',
         '../Core/Resource',
@@ -18,6 +19,7 @@ define([
         defaultValue,
         defined,
         DeveloperError,
+        GeographicProjection,
         GeographicTilingScheme,
         Rectangle,
         Resource,
@@ -198,7 +200,7 @@ define([
                 // 'global-mercator' and 'global-geodetic' profiles. In the gdal2Tiles case, X and Y are always in
                 // geodetic degrees.
                 var isGdal2tiles = tilingSchemeName === 'geodetic' || tilingSchemeName === 'mercator';
-                if (tilingScheme instanceof GeographicTilingScheme || isGdal2tiles) {
+                if (tilingScheme.projection instanceof GeographicProjection || isGdal2tiles) {
                     sw = Cartographic.fromDegrees(swXY.x, swXY.y);
                     ne = Cartographic.fromDegrees(neXY.x, neXY.y);
                 } else {