diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index e1571eaf40c6..577f77fb604f 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -523,12 +523,22 @@ auto CarbonExternalASTSource::FindExternalVisibleDeclsByName(
   // Find the Carbon declaration corresponding to this Clang declaration.
   auto* decl = cast<clang::Decl>(
       const_cast<clang::DeclContext*>(decl_context->getPrimaryContext()));
+  if (isa<clang::FunctionDecl>(decl)) {
+    // Functions don't meaningfully have visible decls, but bail out early since
+    // we can't form a `ClangDeclKey` for a function in the abstract.
+    return false;
+  }
   auto key = SemIR::ClangDeclKey::ForNonFunctionDecl(decl);
   auto decl_id = context_->clang_decls().LookupId(key);
-  CARBON_CHECK(
-      decl_id.has_value(),
-      "The DeclContext should already be associated with a Carbon InstId.");
-  auto decl_context_inst_id = context_->clang_decls().Get(decl_id).inst_id;
+  if (!decl_id.has_value()) {
+    return false;
+  }
+  auto clang_decl = context_->clang_decls().Get(decl_id);
+  if (clang_decl.is_imported) {
+    // This is imported from C++, presumably from a Clang AST file, so it's not
+    // our responsibility to provide its name lookup results.
+    return false;
+  }
 
   llvm::SmallVector<Check::LookupScope> lookup_scopes;
 
@@ -536,7 +546,7 @@ auto CarbonExternalASTSource::FindExternalVisibleDeclsByName(
   // here - completeness should've been checked by clang before this point.
   if (!AppendLookupScopesForConstant(
           *context_, SemIR::LocId::None,
-          context_->constant_values().Get(decl_context_inst_id),
+          context_->constant_values().Get(clang_decl.inst_id),
           SemIR::ConstantId::None, /*extended_scope=*/false, &lookup_scopes)) {
     return false;
   }
diff --git a/toolchain/check/testdata/interop/cpp/modules/mixed_with_export.carbon b/toolchain/check/testdata/interop/cpp/modules/mixed_with_export.carbon
new file mode 100644
index 000000000000..507adf3e1f75
--- /dev/null
+++ b/toolchain/check/testdata/interop/cpp/modules/mixed_with_export.carbon
@@ -0,0 +1,53 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
+// EXTRA-ARGS: --clang-arg=-fmodules --clang-arg=-fmodules-cache-path=%t.cache
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/modules/mixed_with_export.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/modules/mixed_with_export.carbon
+
+// --- module.modulemap
+
+module A {
+  header "a.h"
+  export *
+}
+
+// --- a.h
+
+struct A { void F(); };
+
+struct C { void H(); };
+
+namespace N { void I(); }
+
+// --- b.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "a.h";
+
+class B {
+  extend base: Cpp.A;
+  fn G(self);
+}
+
+inline Cpp '''
+Carbon::B b;
+A *a = &b;
+
+void call(C *c) {
+  a->F();
+
+  b.G();
+
+  c->H();
+
+  N::I();
+}
+''';