diff --git a/docs/handbook/image-file-formats.rst b/docs/handbook/image-file-formats.rst
index d26c0fbae..35ec99ece 100644
--- a/docs/handbook/image-file-formats.rst
+++ b/docs/handbook/image-file-formats.rst
@@ -999,7 +999,7 @@ where applicable:
     The number of times to loop this APNG, 0 indicates infinite looping.
 
 **duration**
-    The time to display this APNG frame (in milliseconds).
+    The time to display this APNG frame (in milliseconds), given as a float.
 
 .. note::