diff --git a/src/vs/workbench/services/themes/electron-browser/workbenchThemeService.ts b/src/vs/workbench/services/themes/electron-browser/workbenchThemeService.ts
index 73a5a877efc8bbefdb43d35a762aa9b920e97f42..f183ab0290e91c1eb3ab11f3e6c70cfa12a72d22 100644
--- a/src/vs/workbench/services/themes/electron-browser/workbenchThemeService.ts
+++ b/src/vs/workbench/services/themes/electron-browser/workbenchThemeService.ts
@@ -546,7 +546,7 @@ export class WorkbenchThemeService implements IWorkbenchThemeService {
 			}
 			let normalizedAbsolutePath = Paths.normalize(Paths.join(extensionFolderPath, theme.path));
 
-			if (normalizedAbsolutePath.indexOf(extensionFolderPath) !== 0) {
+			if (normalizedAbsolutePath.indexOf(Paths.normalize(extensionFolderPath)) !== 0) {
 				collector.warn(nls.localize('invalid.path.1', "Expected `contributes.{0}.path` ({1}) to be included inside extension's folder ({2}). This might make the extension non-portable.", themesExtPoint.name, normalizedAbsolutePath, extensionFolderPath));
 			}
 			let themeData = fromExtensionTheme(theme, normalizedAbsolutePath, extensionData);
@@ -589,7 +589,7 @@ export class WorkbenchThemeService implements IWorkbenchThemeService {
 			}
 			let normalizedAbsolutePath = Paths.normalize(Paths.join(extensionFolderPath, iconTheme.path));
 
-			if (normalizedAbsolutePath.indexOf(extensionFolderPath) !== 0) {
+			if (normalizedAbsolutePath.indexOf(Paths.normalize(extensionFolderPath)) !== 0) {
 				collector.warn(nls.localize('invalid.path.1', "Expected `contributes.{0}.path` ({1}) to be included inside extension's folder ({2}). This might make the extension non-portable.", themesExtPoint.name, normalizedAbsolutePath, extensionFolderPath));
 			}