diff --git a/src/calibre/gui2/comments_editor.py b/src/calibre/gui2/comments_editor.py
index 6e350196c2..7b8538cfbc 100644
--- a/src/calibre/gui2/comments_editor.py
+++ b/src/calibre/gui2/comments_editor.py
@@ -252,7 +252,7 @@ class EditorWidget(QTextEdit, LineEditECM):  # {{{
         f = QFontInfo(font)
         delta = tweaks['change_book_details_font_size_by'] + 1
         if delta:
-            font.setPixelSize(f.pixelSize() + delta)
+            font.setPixelSize(int(f.pixelSize() + delta))
             self.setFont(font)
         f = QFontMetrics(self.font())
         self.em_size = f.horizontalAdvance('m')
diff --git a/src/calibre/gui2/widgets2.py b/src/calibre/gui2/widgets2.py
index e1ec5cbc1d..639c1a7cc7 100644
--- a/src/calibre/gui2/widgets2.py
+++ b/src/calibre/gui2/widgets2.py
@@ -530,7 +530,7 @@ class HTMLDisplay(QTextBrowser):
         f = QFontInfo(font)
         delta = tweaks['change_book_details_font_size_by'] + 1
         if delta:
-            font.setPixelSize(f.pixelSize() + delta)
+            font.setPixelSize(int(f.pixelSize() + delta))
             self.setFont(font)
         self.setFrameShape(QFrame.Shape.NoFrame)
         self.setOpenLinks(False)