diff --git a/conversion/romantoint.go b/conversion/romantoint.go
index 629952320..c6df21a53 100644
--- a/conversion/romantoint.go
+++ b/conversion/romantoint.go
@@ -55,5 +55,9 @@ func RomanToInt(input string) (int, error) {
 	if len(input) > 0 {
 		return 0, errors.New("invalid roman numeral")
 	}
+	// Only standard form in the range 1 to 3999 is supported.
+	if output > 3999 {
+		return 0, errors.New("roman numeral must be in range 1 to 3999")
+	}
 	return output, nil
 }
diff --git a/conversion/romantoint_test.go b/conversion/romantoint_test.go
index 0fd34b2c3..41a1b6a39 100644
--- a/conversion/romantoint_test.go
+++ b/conversion/romantoint_test.go
@@ -43,6 +43,11 @@ func TestRomanToInt(t *testing.T) {
 	if err != nil {
 		t.Errorf("RomanToInt(\"\") returned an error %s", err.Error())
 	}
+
+	// MMMM is 4000, which is outside the supported 1-3999 range.
+	if _, err := RomanToInt("MMMM"); err == nil {
+		t.Error("RomanToInt(MMMM) expected an error")
+	}
 }
 
 func BenchmarkRomanToInt(b *testing.B) {