diff --git a/lib/fbe/award.rb b/lib/fbe/award.rb
index ad3d8cd..1cd00dd 100644
--- a/lib/fbe/award.rb
+++ b/lib/fbe/award.rb
@@ -358,7 +358,12 @@ def set(var, value)
     #   bill.line(50, "for code review")
     def line(value, text)
       return if value.zero?
-      text = text.gsub(/\$\{([a-z_0-9]+)\}/) { |_x| @vars[Regexp.last_match[1].to_sym] }
+      text =
+        text.gsub(/\$\{([a-z_0-9]+)\}/) do |_x|
+          k = Regexp.last_match[1].to_sym
+          raise(Fbe::Error, "Undefined variable '#{k}' used in award text: #{text}") unless @vars.key?(k)
+          @vars[k]
+        end
       @lines << { v: value, t: text }
     end
 
@@ -445,7 +450,12 @@ def intro(text)
     #   bylaw.let(:points, 50)
     #   bylaw.line("award ${points} points")
     def line(line)
-      line = line.gsub(/\$\{([a-z_0-9]+)\}/) { |_x| "**#{@lets[Regexp.last_match[1].to_sym]}**" }
+      line =
+        line.gsub(/\$\{([a-z_0-9]+)\}/) do |_x|
+          k = Regexp.last_match[1].to_sym
+          raise(Fbe::Error, "Undefined variable '#{k}' used in bylaw text: #{line}") unless @lets.key?(k)
+          "**#{@lets[k]}**"
+        end
       @lines << line
     end
 
diff --git a/test/fbe/test_award.rb b/test/fbe/test_award.rb
index 0ce4c15..44a0a30 100644
--- a/test/fbe/test_award.rb
+++ b/test/fbe/test_award.rb
@@ -130,6 +130,16 @@ def test_shorten_when_nothing
     assert_equal('You\'ve earned nothing. ', g, g)
   end
 
+  def test_bill_raises_on_undefined_var
+    a = Fbe::Award.new('(award (give 10 "test ${missing}"))')
+    assert_raises(Fbe::Error) { a.bill }
+  end
+
+  def test_bylaw_raises_on_undefined_var
+    a = Fbe::Award.new('(award (aka (give 10 "points") "${undefined} points"))')
+    assert_raises(Fbe::Error) { a.bylaw }
+  end
+
   def test_division_by_zero_raises_error
     a = Fbe::Award.new('(award (set x (div 10 0)) (give x "test"))')
     assert_raises(Fbe::Error) { a.bill }