diff --git a/lib/fugit/nat.rb b/lib/fugit/nat.rb
index 680e607..abdf74d 100644
--- a/lib/fugit/nat.rb
+++ b/lib/fugit/nat.rb
@@ -642,9 +642,9 @@ def to_crons(opts)
           fail(ArgumentError.new(
             "multiple crons in #{opts[:_s].inspect} - #{@slots.inspect}"))
         elsif multi == true
-          hms.collect { |hm| parse_cron(hm) }
+          hms.collect { |hm| parse_cron(hm, opts) }
         else
-          parse_cron(hms.first)
+          parse_cron(hms.first, opts)
         end
       end
 
@@ -678,7 +678,7 @@ def determine_hms
           .to_a
       end
 
-      def parse_cron(hm)
+      def parse_cron(hm, opts)
 
         a = [
           slot(:second, '0'),
@@ -695,7 +695,32 @@ def parse_cron(hm)
           .collect { |e| e.uniq.sort.collect(&:to_s).join(',') }
           .join(' ')
 
-        Fugit::Cron.parse(s)
+        c = Fugit::Cron.parse(s)
+
+        if opts[:strict]
+          restrict(a, c)
+        else
+          c
+        end
+      end
+
+      # Return nil if the cron is "not strict"
+      #
+      # For example, "0 0/17 * * *" (gh-86) is a perfectly valid
+      # cron string, but makes not much sense when derived via `.parse_nat`
+      # from "every 17 hours".
+      #
+      # It happens here because it's nat being strict, not cron.
+      #
+      def restrict(a, cron)
+
+        if m = ((a[1] && a[1][0]) || '').match(/^(\d+|\*)\/(\d+)$/)
+#p m
+           sla = m[1].to_i
+          return nil unless [ 1, 2, 3, 4, 5, 6, 8, 12 ].include?(sla)
+        end
+
+        cron
       end
 
       def slot(key, default)
diff --git a/spec/nat_spec.rb b/spec/nat_spec.rb
index 8b132f6..6caf40f 100644
--- a/spec/nat_spec.rb
+++ b/spec/nat_spec.rb
@@ -302,15 +302,26 @@
 
     ].each do |input|
 
-      it "rejects (returns nil) for  #{input.inspect}" do
+      it "rejects (returns nil) for #{input.inspect}" do
 
         expect(Fugit::Nat.parse(input)).to eq(nil)
       end
     end
 
-    #context 'strict:' do
-    # TODO
-    #end
+    context 'strict: true' do
+
+      [
+
+        'every 17 hours',
+
+      ].each do |input|
+
+        it "rejects (returns nil) for #{input.inspect}" do
+
+          expect(Fugit::Nat.parse(input, strict: true)).to eq(nil)
+        end
+      end
+    end
   end
 
   describe '.do_parse' do