diff --git a/trees/redblacktree/redblacktree.go b/trees/redblacktree/redblacktree.go
index b012d24..ecc199e 100644
--- a/trees/redblacktree/redblacktree.go
+++ b/trees/redblacktree/redblacktree.go
@@ -64,6 +64,8 @@ func NewWithStringComparator() *Tree {
 func (tree *Tree) Put(key interface{}, value interface{}) {
 	var insertedNode *Node
 	if tree.Root == nil {
+		// Assert key is of comparator's type for initial tree
+		tree.Comparator(key, key)
 		tree.Root = &Node{Key: key, Value: value, color: red}
 		insertedNode = tree.Root
 	} else {