diff --git a/clang-tools-extra/pseudo/test/crash/bad-ucn.c b/clang-tools-extra/pseudo/test/crash/bad-ucn.c
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/pseudo/test/crash/bad-ucn.c
@@ -0,0 +1,4 @@
+// This UCN doesn't encode a valid codepoint.
+// We used to assert while trying to expand UCNs in the token.
+// RUN: clang-pseudo -source=%s
+A\UAAAAAAAA
diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -320,10 +320,8 @@
                             llvm::SmallVectorImpl<char> &Str) {
   char ResultBuf[4];
   char *ResultPtr = ResultBuf;
-  bool Res = llvm::ConvertCodePointToUTF8(Codepoint, ResultPtr);
-  (void)Res;
-  assert(Res && "Unexpected conversion failure");
-  Str.append(ResultBuf, ResultPtr);
+  if (llvm::ConvertCodePointToUTF8(Codepoint, ResultPtr))
+    Str.append(ResultBuf, ResultPtr);
 }
 
 void clang::expandUCNs(SmallVectorImpl<char> &Buf, StringRef Input) {
diff --git a/clang/test/Lexer/unicode.c b/clang/test/Lexer/unicode.c
--- a/clang/test/Lexer/unicode.c
+++ b/clang/test/Lexer/unicode.c
@@ -28,6 +28,9 @@
 
         int _;
 
+extern int X\UAAAAAAAA; // expected-error {{not allowed in an identifier}}
+int Y = '\UAAAAAAAA'; // expected-error {{invalid universal character}}
+
 #ifdef __cplusplus
 
 extern int ༀ;