diff --git a/libc/src/__support/str_to_float.h b/libc/src/__support/str_to_float.h
--- a/libc/src/__support/str_to_float.h
+++ b/libc/src/__support/str_to_float.h
@@ -1180,7 +1180,9 @@
       if (src[index] == '(') {
         size_t left_paren = index;
         ++index;
-        while (isalnum(src[index]))
+        // Apparently it's common for underscores to also be accepted. No idea
+        // why, but it's causing fuzz failures.
+        while (isalnum(src[index]) || src[index] == '_')
           ++index;
         if (src[index] == ')') {
           ++index;
diff --git a/libc/test/src/stdlib/strtof_test.cpp b/libc/test/src/stdlib/strtof_test.cpp
--- a/libc/test/src/stdlib/strtof_test.cpp
+++ b/libc/test/src/stdlib/strtof_test.cpp
@@ -201,4 +201,8 @@
   run_test("NaN(1a)", 7, 0x7fc00000);
   run_test("NaN(asdf)", 9, 0x7fc00000);
   run_test("NaN(1A1)", 8, 0x7fc00000);
+  run_test("NaN(why_does_this_work)", 23, 0x7fc00000);
+  run_test(
+      "NaN(1234567890qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM_)",
+      68, 0x7fc00000);
 }