diff --git a/src/Errors/mode.js b/src/Errors/mode.js
new file mode 100644
index 0000000..1b826cb
--- /dev/null
+++ b/src/Errors/mode.js
@@ -0,0 +1,5 @@
+import { ISicElementIDError } from "./isicElementIDError.js";
+
+export class InvalidModeError extends Error {
+
+}
\ No newline at end of file
diff --git a/src/Pure/format.js b/src/Pure/format.js
index 4cc2804..28f88c4 100644
--- a/src/Pure/format.js
+++ b/src/Pure/format.js
@@ -1,4 +1,6 @@
 import Constants from "./constants/constants.js"
+import { BaseIndexError } from "../Errors/base.js"
+
 
 export default class Format {
 
@@ -21,7 +23,7 @@ export default class Format {
             return "ISic" + padded.slice(0, 6) + "-" + padded.slice(6, 10)    
         }
         else {
-            console.error("Invalid base")
+            throw new BaseIndexError(baseIdx)
         }
         
     }
diff --git a/src/SideEffects/status.js b/src/SideEffects/status.js
index f67fd53..61c32c3 100644
--- a/src/SideEffects/status.js
+++ b/src/SideEffects/status.js
@@ -1,9 +1,10 @@
 import Attrs from "./attrs.js"
 import Elems from "./elems.js"
+import { InvalidModeError } from "../Errors/mode.js"
 
 export default class Status {
     /**
-     * @returns {"compression"|"midpoint"|"error"} 
+     * @returns {"compression"|"midpoint"} 
      */
     static selectionMode = () => {
 
@@ -13,7 +14,7 @@ export default class Status {
             return "midpoint"
         }
 
-        console.error("Invalid selection mode")
-        return "error"
+        // This should never happen
+        throw new InvalidModeError(`Invalid selection mode`)
     }
 }