diff --git a/util/util.go b/util/util.go
index e127f524..03ee4254 100644
--- a/util/util.go
+++ b/util/util.go
@@ -33,6 +33,9 @@ import (
 // you specify 0, you'll get `a`, 25 gives you `z`, and 26 gives you `aa` and so
 // on...
 func NumToAlpha(idx int) string {
+	if idx < 0 {
+		return "" // don't error or produce junk
+	}
 	var mod = idx % 26
 	var div = idx / 26
 	if div > 0 {
diff --git a/util/util_test.go b/util/util_test.go
index 07609f08..8070f52a 100644
--- a/util/util_test.go
+++ b/util/util_test.go
@@ -38,6 +38,7 @@ func TestNumToAlpha(t *testing.T) {
 		{702, "aaa"},
 		{703, "aab"},
 		{63269, "cool"},
+		{-1, ""},
 	}
 
 	for _, test := range numToAlphaTests {