diff --git a/src/PIL/BlpImagePlugin.py b/src/PIL/BlpImagePlugin.py
index e07474621..7b78597b4 100644
--- a/src/PIL/BlpImagePlugin.py
+++ b/src/PIL/BlpImagePlugin.py
@@ -417,9 +417,11 @@ class BLP2Decoder(_BLPBaseDecoder):
         self.set_as_raw(bytes(data))
 
 
-Image.register_open(
-    BlpImageFile.format, BlpImageFile, lambda p: p[:4] in (b"BLP1", b"BLP2")
-)
+def _accept(prefix):
+    return prefix[:4] in (b"BLP1", b"BLP2")
+
+
+Image.register_open(BlpImageFile.format, BlpImageFile, _accept)
 Image.register_extension(BlpImageFile.format, ".blp")
 
 Image.register_decoder("BLP1", BLP1Decoder)
diff --git a/src/PIL/IcnsImagePlugin.py b/src/PIL/IcnsImagePlugin.py
index ca6a0adad..a38f2fc5c 100644
--- a/src/PIL/IcnsImagePlugin.py
+++ b/src/PIL/IcnsImagePlugin.py
@@ -359,7 +359,11 @@ def _save(im, fp, filename):
                 fp.write(f.read())
 
 
-Image.register_open(IcnsImageFile.format, IcnsImageFile, lambda x: x[:4] == b"icns")
+def _accept(prefix):
+    return prefix[:4] == b"icns"
+
+
+Image.register_open(IcnsImageFile.format, IcnsImageFile, _accept)
 Image.register_extension(IcnsImageFile.format, ".icns")
 
 if sys.platform == "darwin":