diff --git a/libdispatch/dattput.c b/libdispatch/dattput.c
index 712375d6e..2a87699c9 100644
--- a/libdispatch/dattput.c
+++ b/libdispatch/dattput.c
@@ -61,9 +61,10 @@ nc_put_att_string(int ncid, int varid, const char *name,
 \ingroup attributes
 Write a text attribute.
 
-Add or change a text attribute. If this attribute is new,
-or if the space required to store the attribute is greater than
-before, the netCDF dataset must be in define mode.
+Add or change a text attribute. If this attribute is new, or if the
+space required to store the attribute is greater than before, the
+netCDF dataset must be in define mode for classic formats (or
+netCDF-4/HDF5 with NC_CLASSIC_MODEL).
 
 Although it's possible to create attributes of all types, text and
 double attributes are adequate for most purposes.