diff --git a/lib/Demangle/MicrosoftDemangle.cpp b/lib/Demangle/MicrosoftDemangle.cpp
index 14c6e1e5ef5..4942e0e3655 100644
--- a/lib/Demangle/MicrosoftDemangle.cpp
+++ b/lib/Demangle/MicrosoftDemangle.cpp
@@ -1154,8 +1154,11 @@ static unsigned countEmbeddedNulls(const uint8_t *StringBytes,
   return Result;
 }
 
+// A mangled (non-wide) string literal stores the total length of the string it
+// refers to (passed in NumBytes), and it contains up to 32 bytes of actual text
+// (passed in StringBytes, NumChars).
 static unsigned guessCharByteSize(const uint8_t *StringBytes, unsigned NumChars,
-                                  unsigned NumBytes) {
+                                  uint64_t NumBytes) {
   assert(NumBytes > 0);
 
   // If the number of bytes is odd, this is guaranteed to be a char string.
diff --git a/test/Demangle/ms-string-literals.test b/test/Demangle/ms-string-literals.test
index 0e9d1edea8a..43c03c1c51e 100644
--- a/test/Demangle/ms-string-literals.test
+++ b/test/Demangle/ms-string-literals.test
@@ -792,3 +792,8 @@
 
 ??_C@_07LJGFEJEB@D3?$CC?$BB?$AA?$AA?$AA?$AA@)
 ; CHECK: U"\x11223344"
+
+; This has a string length of 0x6_0000_0000, so it's 0 if treated as a 32-bit
+; number. (In practice, 24GiB large string literals should be rare.)
+??_C@_0GAAAAAAAA@GPLEPFHO@01234567890123456789012345678901@
+; CHECK: "01234567890123456789012345678901"...