diff --git a/tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp b/tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp
index 6dd5014f9e4..24324ca986f 100644
--- a/tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp
+++ b/tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp
@@ -284,6 +284,8 @@ swiftparse_client_node_t SynParser::parse(const char *source) {
   ParserUnit PU(SM, SourceFileKind::Main, bufID, langOpts,
                 "syntax_parse_module", std::move(parseActions),
                 /*SyntaxCache=*/nullptr);
+  // Evaluating pound conditions may lead to unknown syntax.
+  PU.getParser().State->PerformConditionEvaluation = false;
   std::unique_ptr<SynParserDiagConsumer> pConsumer;
   if (DiagHandler) {
     pConsumer = llvm::make_unique<SynParserDiagConsumer>(*this, bufID);