Index: src/Concurrency/Corun.cpp
===================================================================
--- src/Concurrency/Corun.cpp	(revision eb779d5ca94c2de1db899969438189f4b2c05b4b)
+++ src/Concurrency/Corun.cpp	(revision ee9ad4067d87f93f95150db093d5d9dc0266eecd)
@@ -42,5 +42,7 @@
 
     Stmt * postvisit( const CorunStmt * stmt ) {
-        assert( runnerBlockDecl );
+        if ( !runnerBlockDecl ) 
+            SemanticError( stmt->location, "To use corun statements add #include <cofor.hfa>\n" );
+
         if ( !stmt->stmt )
             return nullptr;