diff --git a/doc/src/Errors_details.rst b/doc/src/Errors_details.rst
index b63d80a9ae..2a08a308f4 100644
--- a/doc/src/Errors_details.rst
+++ b/doc/src/Errors_details.rst
@@ -13,15 +13,27 @@ discussions of such cases.
 Unknown identifier in data file
 -------------------------------
 
-This error happens when LAMMPS encounters a line of text in an unexpected format
-while reading a data file. This is most commonly cause by inconsistent header and
-section data.  The header section informs LAMMPS how many entries or lines are expected in the
-various sections (like Atoms, Masses, Pair Coeffs, *etc.*\ ) of the data file.
-If there is a mismatch, LAMMPS will either keep reading beyond the end of a section
-or stop reading before the section has ended.
+This error happens when LAMMPS encounters a line of text with an
+unexpected keyword while :doc:`reading a data file <read_data>`.  This
+would be either header keywords or section header keywords.  This is
+most commonly due to a mistyped keyword or due to a keyword that is
+inconsistent with the :doc:`atom style <atom_style>` used.
 
-Such a mismatch can happen unexpectedly when the first line of the data
-is *not* a comment as required by the format.  That would result in
-LAMMPS expecting, for instance, 0 atoms because the "atoms" header line
-is treated as a comment.
+The header section informs LAMMPS how many entries or lines are expected
+in the various sections (like Atoms, Masses, Pair Coeffs, *etc.*\ ) of
+the data file.  If there is a mismatch, LAMMPS will either keep reading
+beyond the end of a section or stop reading before the section has
+ended.  In that case the next line will not contain a recognized keyword.
 
+Such a mismatch can also happen when the first line of the data
+is *not* a comment as required by the format, but a line with a valid
+header keyword.  That would result in LAMMPS expecting, for instance,
+0 atoms because the "atoms" header line is the first line and thus
+treated as a comment.
+
+Another possibility to trigger this error is to have a keyword in the
+data file that corresponds to a fix (e.g. :doc:`fix cmap <fix_cmap>`)
+but the :doc:`read_data <read_data>` command is missing the (optional)
+arguments that identify the fix and the header keyword and section
+keyword or those arguments are inconsistent with the keywords in the
+data file.
diff --git a/src/read_data.cpp b/src/read_data.cpp
index 657369d5d0..36bbce605f 100644
--- a/src/read_data.cpp
+++ b/src/read_data.cpp
@@ -1360,7 +1360,8 @@ void ReadData::header(int firstpass)
   // check that exiting string is a valid section keyword
 
   parse_keyword(1);
-  if (!is_data_section(keyword)) error->all(FLERR, "Unknown identifier in data file: {}", keyword);
+  if (!is_data_section(keyword))
+    error->all(FLERR, "Unknown identifier in data file: {}{}", keyword, utils::errorurl(1));
 
   // error checks on header values
   // must be consistent with atom style and other header values