diff --git a/pngzop b/pngzop
index d4a336f50..06876bd8d 100755
--- a/pngzop
+++ b/pngzop
@@ -22,7 +22,7 @@
 
 # Usage:
 # pngzop [-b|--blacken] [-d|--directory dir] [-e|--extension ext] *.png
-#        (to overwrite the input, use "-e png")
+#        (to overwrite the input, use "-e .png")
 
 # Input: *.png
 # Output: *_pngzop.png
diff --git a/pngzop_README.txt b/pngzop_README.txt
index 370bc7030..a7db5f668 100644
--- a/pngzop_README.txt
+++ b/pngzop_README.txt
@@ -14,7 +14,7 @@
   pngzop [-b|--blacken] [-d|--directory dir] [-e|--extension ext] *.png
 
   Input: *.png
-  Output (default): *_pngzop.png (to overwrite the input, use "-e png")
+  Output (default): *_pngzop.png (to overwrite the input, use "-e .png")
 
   Pngzop does the following: