diff --git a/hill.lisp b/hill.lisp
index 10f0746..544c4d8 100644
--- a/hill.lisp
+++ b/hill.lisp
@@ -146,7 +146,7 @@ return a cons of (x . y) such char x * N1 + y * N2 = GCD."
 integer i such that (N * i) mod M = 1."
   (destructuring-bind (gcd (x . y))
       (multiple-value-list (extended-gcd (if (minusp n)
-                                             (+ 26 n)
+                                             (+ m n)
                                              n)
                                          m))
     (declare (ignorable y))
diff --git a/shift-affine.lisp b/shift-affine.lisp
index fde6706..0f1c768 100644
--- a/shift-affine.lisp
+++ b/shift-affine.lisp
@@ -62,7 +62,7 @@ return a cons of (x . y) such char x * N1 + y * N2 = GCD."
 integer i such that (N * i) mod M = 1."
   (destructuring-bind (gcd (x . y))
       (multiple-value-list (extended-gcd (if (minusp n)
-                                             (+ 26 n)
+                                             (+ m n)
                                              n)
                                          m))
     (declare (ignorable y))