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
@@ -783,14 +783,28 @@
       src += 3;
       BitsType NaNMantissa = 0;
       if (*src == '(') {
-        char *tempSrc = 0;
-        if (isdigit(*(src + 1)) || *(src + 1) == ')') {
-          NaNMantissa = strtointeger<BitsType>(src + 1, &tempSrc, 0);
-          if (*tempSrc != ')') {
-            NaNMantissa = 0;
-          } else {
-            src = tempSrc + 1;
+        const char *parenStart = src;
+        ++src;
+        while (isalnum(*src)) {
+          ++src;
+        }
+        if (*src == ')') {
+          ++src;
+          char *tempSrc = 0;
+          if (isdigit(*(parenStart + 1))) {
+            // This is to prevent errors when BitsType is larger than 64 bits,
+            // since strtointeger only supports up to 64 bits. This is actually
+            // more than is required by the specification, which says for the
+            // input type "NAN(n-char-sequence)" that "the meaning of
+            // the n-char sequence is implementation-defined."
+            NaNMantissa = static_cast<BitsType>(
+                strtointeger<uint64_t>(parenStart + 1, &tempSrc, 0));
+            if (*tempSrc != ')') {
+              NaNMantissa = 0;
+            }
           }
+        } else {
+          src = parenStart;
         }
       }
       NaNMantissa |= fputil::FloatProperties<T>::quietNaNMask;
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
@@ -160,4 +160,5 @@
   runTest("NaN()", 5, 0x7fc00000);
   runTest("NaN(1234)", 9, 0x7fc004d2);
   runTest("NaN( 1234)", 3, 0x7fc00000);
+  runTest("NaN(asdf)", 9, 0x7fc00000);
 }