diff --git a/sopel_modules/spicemanip/__init__.py b/sopel_modules/spicemanip/__init__.py
index 3fefa48..b507df7 100644
--- a/sopel_modules/spicemanip/__init__.py
+++ b/sopel_modules/spicemanip/__init__.py
@@ -320,7 +320,7 @@ class Spicemanip():
             return ' '.join(inputs)
         return ', '.join(str(x) for x in inputs)
 
-    def is_digit(n):
+    def is_digit(self, n):
         try:
             int(n)
             return True