diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 2950ff1fd..a7ad580d2 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -168,7 +168,7 @@ struct ExprVar : Expr
        the set stored in the environment that is `level` levels up
        from the current one.*/
     Level level;
-    Displacement displ;
+    Displacement displ = 0;
 
     ExprVar(Symbol name) : name(name) { };
     ExprVar(const PosIdx & pos, Symbol name) : pos(pos), name(name) { };
@@ -242,7 +242,7 @@ struct ExprAttrs : Expr
         Kind kind;
         Expr * e;
         PosIdx pos;
-        Displacement displ; // displacement
+        Displacement displ = 0; // displacement
         AttrDef(Expr * e, const PosIdx & pos, Kind kind = Kind::Plain)
             : kind(kind), e(e), pos(pos) { };
         AttrDef() { };