diff --git a/src/program.cc b/src/program.cc
index 8b7b4bc136..269c738ce7 100644
--- a/src/program.cc
+++ b/src/program.cc
@@ -47,6 +47,7 @@ Program::Program(ProgramBuilder&& builder) {
     TypeDeterminer td(&builder);
     if (!td.Determine()) {
       diagnostics_.add_error(td.error());
+      is_valid_ = false;
     }
   }
 
@@ -60,7 +61,7 @@ Program::Program(ProgramBuilder&& builder) {
       builder.AST().GlobalVariables());
   sem_ = std::move(builder.Sem());
   symbols_ = std::move(builder.Symbols());
-  diagnostics_ = std::move(builder.Diagnostics());
+  diagnostics_.add(std::move(builder.Diagnostics()));
   builder.MarkAsMoved();
 
   if (!is_valid_ && !diagnostics_.contains_errors()) {
diff --git a/src/program_test.cc b/src/program_test.cc
index 75dde4925f..e958c1b56c 100644
--- a/src/program_test.cc
+++ b/src/program_test.cc
@@ -19,6 +19,7 @@
 
 #include "gmock/gmock.h"
 #include "src/ast/function.h"
+#include "src/ast/return_statement.h"
 #include "src/ast/test_helper.h"
 #include "src/ast/variable.h"
 #include "src/type/alias_type.h"
@@ -126,6 +127,18 @@ TEST_F(ProgramTest, IsValid_Invalid_Function) {
   EXPECT_FALSE(program.IsValid());
 }
 
+TEST_F(ProgramTest, IsValid_Invalid_UnknownVar) {
+  Func("main", ast::VariableList{}, nullptr,
+       ast::StatementList{
+           create<ast::ReturnStatement>(Expr("unknown_ident")),
+       },
+       ast::FunctionDecorationList{});
+
+  Program program(std::move(*this));
+  EXPECT_FALSE(program.IsValid());
+  EXPECT_NE(program.Diagnostics().count(), 0u);
+}
+
 TEST_F(ProgramTest, IsValid_GeneratesError) {
   AST().AddGlobalVariable(nullptr);