diff --git a/adafruit_bitmap_font/bdf.py b/adafruit_bitmap_font/bdf.py
index 256e239..7c6e830 100644
--- a/adafruit_bitmap_font/bdf.py
+++ b/adafruit_bitmap_font/bdf.py
@@ -97,7 +97,7 @@ def load_glyphs(self, code_points):
             remaining = code_points
         else:
             remaining = set(code_points)
-        for code_point in remaining:
+        for code_point in remaining.copy():
             if code_point in self._glyphs and self._glyphs[code_point]:
                 remaining.remove(code_point)
         if not remaining: