diff --git a/booklore-ui/src/app/epub-viewer/component/epub-viewer.component.ts b/booklore-ui/src/app/epub-viewer/component/epub-viewer.component.ts
index 43bd73639..aa0d818f6 100644
--- a/booklore-ui/src/app/epub-viewer/component/epub-viewer.component.ts
+++ b/booklore-ui/src/app/epub-viewer/component/epub-viewer.component.ts
@@ -165,12 +165,12 @@ export class EpubViewerComponent implements OnInit, OnDestroy {
   }
 
   increaseFontSize(): void {
-    this.fontSize = Math.min(this.fontSize + 10, FALLBACK_EPUB_SETTINGS.maxFontSize);
+    this.fontSize = Math.min(Number(this.fontSize) + 10, FALLBACK_EPUB_SETTINGS.maxFontSize);
     this.updateFontSize();
   }
 
   decreaseFontSize(): void {
-    this.fontSize = Math.max(this.fontSize - 10, FALLBACK_EPUB_SETTINGS.minFontSize);
+    this.fontSize = Math.max(Number(this.fontSize) - 10, FALLBACK_EPUB_SETTINGS.minFontSize);
     this.updateFontSize();
   }