Index: src/Concurrency/Keywords.cc
===================================================================
--- src/Concurrency/Keywords.cc	(revision afe9e454560ea0dd21509c0e11af7e348b940597)
+++ src/Concurrency/Keywords.cc	(revision a494d1055b2782f956b9b766d4b211dbc716fc5a)
@@ -422,6 +422,7 @@
 			;
 		else if ( auto param = isMainFor( decl, cast_target ) ) {
-			// This should never trigger.
-			assert( vtable_decl );
+			if ( !vtable_decl ) {
+				SemanticError( decl, context_error );
+			}
 			// Should be safe because of isMainFor.
 			StructInstType * struct_type = static_cast<StructInstType *>(