diff --git a/libc/test/src/ctype/isalpha_test.cpp b/libc/test/src/ctype/isalpha_test.cpp
--- a/libc/test/src/ctype/isalpha_test.cpp
+++ b/libc/test/src/ctype/isalpha_test.cpp
@@ -6,17 +6,22 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <stdio.h>
+
 #include "src/ctype/isalpha.h"
 
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcIsAlpha, DefaultLocale) {
   // Loops through all characters, verifying that letters return a
-  // non-zero integer and everything else returns zero.
-  for (int ch = 0; ch < 255; ++ch) {
-    if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z'))
+  // non-zero integer and everything else including EOF returns zero.
+  for (int ch = 0; ch <= 255; ++ch) {
+    if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z')) {
       EXPECT_NE(__llvm_libc::isalpha(ch), 0);
-    else
+    } else if (ch == EOF) {
+      EXPECT_EQ(__llvm_libc::isalpha(ch), 0);
+    } else {
       EXPECT_EQ(__llvm_libc::isalpha(ch), 0);
+    }
   }
 }