diff --git a/itemdef.go b/itemdef.go
index 56baa7f..145ed9f 100644
--- a/itemdef.go
+++ b/itemdef.go
@@ -116,6 +116,9 @@ func bestCap(defs [][]byte, capabs []*ToolCapabs) *ItemDef {
 		}
 	}
 
+	if bestK >= len(defs) {
+		return &ItemDef{}
+	}
 	return &ItemDef{data: defs[bestK]}
 }