diff --git a/src/mapping.cpp b/src/mapping.cpp
index 2301a8b..94b89b8 100644
--- a/src/mapping.cpp
+++ b/src/mapping.cpp
@@ -36,7 +36,7 @@ bool ascii_has_upper_case(char* input, size_t length) {
   auto broadcast = [](uint8_t v) -> uint64_t { return 0x101010101010101 * v; };
   uint64_t broadcast_80 = broadcast(0x80);
   uint64_t broadcast_Ap = broadcast(128 - 'A');
-  uint64_t broadcast_Zp = broadcast(128 - 'Z');
+  uint64_t broadcast_Zp = broadcast(128 - 'Z' - 1);
   size_t i = 0;
 
   uint64_t runner{0};
@@ -58,7 +58,7 @@ void ascii_map(char* input, size_t length) {
   auto broadcast = [](uint8_t v) -> uint64_t { return 0x101010101010101 * v; };
   uint64_t broadcast_80 = broadcast(0x80);
   uint64_t broadcast_Ap = broadcast(128 - 'A');
-  uint64_t broadcast_Zp = broadcast(128 - 'Z');
+  uint64_t broadcast_Zp = broadcast(128 - 'Z' - 1);
   size_t i = 0;
 
   for (; i + 7 < length; i += 8) {
diff --git a/tests/fixtures/to_ascii_alternating.txt b/tests/fixtures/to_ascii_alternating.txt
index f69f2ee..66b3f33 100644
--- a/tests/fixtures/to_ascii_alternating.txt
+++ b/tests/fixtures/to_ascii_alternating.txt
@@ -1,3 +1,5 @@
+AMAZON
+amazon
 aa--
 aa--
 a†--