diff --git a/lang/structs.go b/lang/structs.go
index 3fccc088..a41e8bde 100644
--- a/lang/structs.go
+++ b/lang/structs.go
@@ -5583,6 +5583,18 @@ func (obj *ExprIf) Unify() ([]interfaces.Invariant, error) {
 	}
 	invariants = append(invariants, branchesInvar)
 
+	// the two branches must match the type of the whole expression
+	thenInvar := &unification.EqualityInvariant{
+		Expr1: obj,
+		Expr2: obj.ThenBranch,
+	}
+	invariants = append(invariants, thenInvar)
+	elseInvar := &unification.EqualityInvariant{
+		Expr1: obj,
+		Expr2: obj.ElseBranch,
+	}
+	invariants = append(invariants, elseInvar)
+
 	return invariants, nil
 }