diff --git a/src/cron/expression.v b/src/cron/expression.v
index 71ee9a1..60e1b74 100644
--- a/src/cron/expression.v
+++ b/src/cron/expression.v
@@ -147,6 +147,18 @@ fn parse_range(s string, min int, max int, mut bitv []bool) ? {
 	}
 }
 
+fn bitv_to_ints(bitv []bool, min int) []int {
+	mut out := []int{}
+
+	for i in 0..bitv.len {
+		if bitv[i] {
+			out << min + i
+		}
+	}
+
+	return out
+}
+
 fn parse_part(s string, min int, max int) ?[]int {
 	mut bitv := []bool{init: false, len: max - min + 1}
 
@@ -154,15 +166,7 @@ fn parse_part(s string, min int, max int) ?[]int {
 		parse_range(range, min, max, mut bitv) ?
 	}
 
-	mut out := []int{}
-
-	for i in 0..max + 1 {
-		if bitv[i] {
-			out << min + i
-		}
-	}
-
-	return out
+	return bitv_to_ints(bitv, min)
 }
 
 // min hour day month day-of-week
diff --git a/src/cron/expression_parse_test.v b/src/cron/expression_parse_test.v
new file mode 100644
index 0000000..7a4974d
--- /dev/null
+++ b/src/cron/expression_parse_test.v
@@ -0,0 +1,65 @@
+module cron
+
+// parse_range_error returns the returned error message. If the result is '',
+// that means the function didn't error.
+fn parse_range_error(s string, min int, max int) string {
+	mut bitv := []bool{init: false, len: max - min + 1}
+
+	parse_range(s, min, max, mut bitv) or {
+		return err.msg
+	}
+
+	return ''
+}
+
+// =====parse_range=====
+fn test_parse_star_range() ? {
+	mut bitv := []bool{init: false, len: 6}
+	parse_range('*', 0, 5, mut bitv) ?
+
+	assert bitv == [true, true, true, true, true, true]
+}
+
+fn test_parse_number() ? {
+	mut bitv := []bool{init: false, len: 6}
+	parse_range('4', 0, 5, mut bitv) ?
+
+	assert bitv_to_ints(bitv, 0) == [4]
+}
+
+fn test_parse_number_too_large() ? {
+	assert parse_range_error('10', 0, 6) == 'Out of range.'
+}
+
+fn test_parse_number_too_small() ? {
+	assert parse_range_error('0', 2, 6) == 'Out of range.'
+}
+
+fn test_parse_step_star() ? {
+	mut bitv := []bool{init: false, len: 21}
+	parse_range('*/4', 0, 20, mut bitv) ?
+
+	assert bitv_to_ints(bitv, 0) == [0, 4, 8, 12, 16, 20]
+}
+
+fn test_parse_step_star_too_large() ? {
+	assert parse_range_error('*/21', 0, 20) == 'Step too large.'
+}
+
+fn test_parse_step_zero() ? {
+	assert parse_range_error('*/0', 0, 20) == 'Step size zero not allowed.'
+}
+
+fn test_parse_step_number() ? {
+	mut bitv := []bool{init: false, len: 21}
+	parse_range('5/4', 0, 20, mut bitv) ?
+	assert bitv_to_ints(bitv, 0) == [5, 9, 13, 17]
+}
+
+fn test_parse_step_number_too_large() ? {
+	assert parse_range_error('10/4', 0, 5) == 'Out of range.'
+}
+
+fn test_parse_step_number_too_small() ? {
+	assert parse_range_error('2/4', 5, 10) == 'Out of range.'
+}
diff --git a/src/cron/expression_test.v b/src/cron/expression_test.v
deleted file mode 100644
index 2d58b15..0000000
--- a/src/cron/expression_test.v
+++ /dev/null
@@ -1,42 +0,0 @@
-module cron
-
-// =====parse_range=====
-fn test_parse_star_range() ? {
-	assert parse_range('*', 0, 5) ? == [0, 1, 2, 3, 4, 5]
-}
-
-fn test_parse_number() ? {
-	assert parse_range('4', 0, 5) ? == [4]
-}
-
-fn test_parse_number_too_large() ? {
-	assert parse_range('10', 0, 6) ? == [6]
-}
-
-fn test_parse_number_too_small() ? {
-	assert parse_range('0', 2, 6) ? == [2]
-}
-
-fn test_parse_step_star() ? {
-	assert parse_range('*/4', 0, 20) ? == [0, 4, 8, 12, 16, 20]
-}
-
-fn test_parse_step_star_too_large() ? {
-	assert parse_range('*/21', 0, 20) ? == [0]
-}
-
-fn test_parse_step_zero() ? {
-	assert parse_range('*/0', 0, 20) ? == []
-}
-
-fn test_parse_step_number() ? {
-	assert parse_range('5/4', 0, 20) ? == [5, 9, 13, 17]
-}
-
-fn test_parse_step_number_too_large() ? {
-	assert parse_range('10/4', 0, 5) ? == [5]
-}
-
-fn test_parse_step_number_too_small() ? {
-	assert parse_range('2/4', 5, 10) ? == [5, 9]
-}