Index: lib/ubsan/ubsan_handlers.cc
===================================================================
--- lib/ubsan/ubsan_handlers.cc
+++ lib/ubsan/ubsan_handlers.cc
@@ -314,12 +314,14 @@
                   sizeof(FilenameOrTypeDescriptor));
 
   // Heuristic: For float_cast_overflow, the TypeKind will be either TK_Integer
-  // (0x0) or TK_Float (0x1). Adding both bytes will be 0 or 1 (for BE or LE).
-  // If it were a filename, adding two printable characters will not yield such
-  // a value.
+  // (0x0), TK_Float (0x1) or TK_Unknown (0xff). If both types are known,
+  // adding both bytes will be 0 or 1 (for BE or LE). If it were a filename,
+  // adding two printable characters will not yield such a value. Otherwise,
+  // if one of them is 0xff, this is probably TK_Unknown type descriptor.
   u16 MaybeFromTypeKind =
       FilenameOrTypeDescriptor[0] + FilenameOrTypeDescriptor[1];
-  return MaybeFromTypeKind < 2;
+  return MaybeFromTypeKind < 2 || FilenameOrTypeDescriptor[0] == 0xff ||
+         FilenameOrTypeDescriptor[1] == 0xff;
 }
 
 static void handleFloatCastOverflow(void *DataPtr, ValueHandle From,