diff --git a/Sources/SwiftParser/Declarations.swift b/Sources/SwiftParser/Declarations.swift
index 9506a609a78..157f316fa90 100644
--- a/Sources/SwiftParser/Declarations.swift
+++ b/Sources/SwiftParser/Declarations.swift
@@ -916,6 +916,10 @@ extension Parser {
               )
             )
           }
+        #if RESILIENT_LIBRARIES
+        @unknown default:
+          fatalError()
+        #endif
         }
 
         keepGoing = self.consume(if: .comma)