diff --git a/.rubocop.yml b/.rubocop.yml
index 0def896e..065e92d7 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -97,6 +97,7 @@ Elegant/GoodMethodName:
     - search_commits
     - search_issues
     - sort_by
+    - sorted_values
     - timeout_unaware
     - total_commits
     - total_commits_pushed
diff --git a/lib/fbe/iterate.rb b/lib/fbe/iterate.rb
index d702a652..54a0e6df 100644
--- a/lib/fbe/iterate.rb
+++ b/lib/fbe/iterate.rb
@@ -203,6 +203,9 @@ def sort_by(prop)
     raise(Fbe::Error, 'Cannot set sort field to nil') if prop.nil?
     raise(Fbe::Error, 'Sort field must be a String') unless prop.is_a?(String)
     raise(Fbe::Error, 'Sort field cannot be empty') if prop.empty?
+    unless prop.match?(/\A[_a-z][a-zA-Z0-9_]*\z/)
+      raise(Fbe::Error, "Wrong sort field format '#{prop}', use [_a-z][a-zA-Z0-9_]*")
+    end
     @sorting = prop
   end
 
@@ -316,9 +319,7 @@ def over # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metric
         end
         nxt =
           if @sorting
-            values[repo] ||= @fb.query(@query).each(
-              @fb, before: before[repo], repository: repo
-            ).filter_map { _1[@sorting]&.first }.uniq.sort!.each
+            values[repo] ||= sorted_values(repo, before[repo])
             begin
               values[repo].next
             rescue StopIteration
@@ -379,4 +380,21 @@ def over # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metric
       end
     end
   end
+
+  private
+
+  # Fetches and sorts the values of the +sort_by+ field for one repository.
+  #
+  # @param [Integer] repo The repository ID
+  # @param [Object] since The +$before+ value to run the query with
+  # @return [Enumerator] The sorted values, ready for +.next+
+  # @raise [Fbe::Error] If matched facts exist but none of them carry the sort field
+  def sorted_values(repo, since)
+    raw = @fb.query(@query).each(@fb, before: since, repository: repo).to_a
+    sorted = raw.filter_map { _1[@sorting]&.first }.uniq
+    if sorted.empty? && !raw.empty?
+      raise(Fbe::Error, "None of the #{raw.size} matched facts in ##{repo} carry the '#{@sorting}' property")
+    end
+    sorted.sort!.each
+  end
 end
diff --git a/test/fbe/test_iterate.rb b/test/fbe/test_iterate.rb
index 3abbdf5c..257eea4f 100644
--- a/test/fbe/test_iterate.rb
+++ b/test/fbe/test_iterate.rb
@@ -579,6 +579,42 @@ def test_sort_by_configuration # rubocop:disable Metrics/AbcSize
     assert_equal(25, fb.query('(eq what "iterate")').each.to_a.first.marker)
   end
 
+  def test_sort_by_rejects_a_bad_format
+    ex =
+      assert_raises(Fbe::Error) do
+        Fbe.iterate(
+          fb: Fbe.fb(fb: Factbase.new, global: {}, options: Judges::Options.new, loog: Loog::NULL),
+          loog: Loog::NULL, options: Judges::Options.new, global: {}, epoch: Time.now, kickoff: Time.now
+        ) do
+          sort_by('9number')
+        end
+      end
+    assert_match(/Wrong sort field format/, ex.message)
+  end
+
+  def test_sort_by_reports_a_misspelled_field_instead_of_looking_exhausted
+    opts = Judges::Options.new(['repositories=foo/bar', 'testing=true'])
+    global = {}
+    fb = Fbe.fb(fb: Factbase.new, global:, options: opts, loog: Loog::NULL)
+    fb.insert.then do |f|
+      f.where = 'github'
+      f.what = 'judge'
+      f.repository = 3_861_188_962
+      f.number = 1
+    end
+    ex =
+      assert_raises(Fbe::Error) do
+        Fbe.iterate(fb:, loog: Loog::NULL, options: opts, global:, epoch: Time.now, kickoff: Time.now) do
+          as('marker')
+          sort_by('numero')
+          by("(and (eq repository $repository) (eq where 'github') (eq what 'judge'))")
+          repeats(1)
+          over { |_repository, n| n }
+        end
+      end
+    assert_match(/carry the 'numero' property/, ex.message)
+  end
+
   def test_custom_since
     opts = Judges::Options.new(['repositories=foo/bar', 'testing=true'])
     fb = Fbe.fb(fb: Factbase.new, global: {}, options: opts, loog: Loog::NULL)