diff --git a/src/ast/ast.h b/src/ast/ast.h
index 277e9120b..1e33cc6d2 100644
--- a/src/ast/ast.h
+++ b/src/ast/ast.h
@@ -522,7 +522,7 @@ public:
 /**
    The ids of expressions and declarations are in different ranges. 
 */
-const unsigned c_first_decl_id = (1 << 31);
+const unsigned c_first_decl_id = (1u << 31);
 
 /**
    \brief Superclass for function declarations and sorts.